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For the same reason we "hang up" our phones.
When a layout designer saves a design, they often say "stream out" whereas in most software, such as Word or Powerpoint, this is usually simply called saving the file. As an aside, it is funny that the icon for saving a file is a floppy disk which anyone aged under about 20 has probably never seen. Plus look at your phone, at the icon to make a call. When did a phone last look like that? It is not just layout designers who say "stream out". As I said yesterday in the blog about Virtuoso Advance Node, I met with Jeremiah Cessna and he says it all the time too.
In the same way, the "stream out" terminology is archaic and comes from a long time ago. Way back before Cadence existed, before even SDA and ECAD, the ingredients of Cadence, existed, EDA consisted of layout and simulation. The biggest of the layout companies was Calma who produced a system called the Graphical Design System. The hardware was a re-badged Data General minicomputer (speaking of which, if you have not read it, read Tracy Kidder's The Soul of a New Machine about developing the successor). The system was called GDSII (pronounced G-D-S-two), was introduced in 1978, and became widespread. An earlier system just called GDS dated to 1971; I think it was mainframe-based but even Wikipedia has a blank placeholder for it and before my time so I cannot add any color.
The system had a disk drive but the general mode of use was to keep designs on magnetic tape. The layout designer would load her (they were mostly women) design from the magnetic tape onto the disk, do their work, and then save the design back to tape. The format that the design was stored in was known as GDSII stream format, and so saving the design back to tape was called "stream out", probably the menu wording, too. The primary reason for doing things this way was that these systems pre-dated cheap local area networks. In a large semiconductor company, there would be several GDSII systems, but the layout designers did not want to have to wait for a particular one to be available, as would be the case if they left the data on the disk. Although in that era, the disk packs were exchangeable but the packs themselves were too expensive to use in that way.
You have probably heard of GDSII stream format, because it became the dominant way that layout information was moved between EDA design tools and is still widely used today almost 40 years later. It became the industry standard but in fact it is not officially an open standard, but a proprietary one. It was developed by Calma and then moved to GE (when they acquired the company that had acquired Calma) then to Valid (when they acquired the Calma division from GE) and then to Cadence when they acquired Valid in 1991.
In the early days, Calma really did consider it proprietary. One of my first jobs when I arrived in the US in 1982 was to write a program to read it. At VLSI Technology, we had been given a microcontroller design in GDSII format, and although we had access to a GDSII system (so we could load it up), that didn't give us a way to get the design into our own design environment that used CIF (Caltech Intermediate Format) for layout. The format is binary so simply looking at what was on the tape didn't help me much. However, eventually we discovered that there was a file on each GDSII system that described the format (it was just two pages long, complete with plenty of copyright warnings). Once we got someone to print it out it only took about a day to have the microcontroller in VLSItools. To this day I'm not sure of the legality of what we did (but I work for Cadence and we own the standard so I think I'm safe now!).
The GDSII stream format and the term "stream out" have survived over the years even though the original computers and magnetic tape in general is obsolete. Only recently has the Oasis (Open Artwork Interchange Standard) standard started to get traction. We have other terms that live on, such as "hanging up" the phone that I mentioned at the start. It is a long time since anyone actually hung the earpiece on a hook to disconnect the call.
In the semiconductor world, we have another term like this, "tapeout". Back in the day of Calma systems, when a design needed to be transferred into manufacturing, a special program then needed to be run to fracture the design into rectangles so that a specialized pattern generator machine (PG) could make the mask. The design would be transferred after fracturing on a tape. So taping the design out really did mean writing a tape, the PG Tape, and getting it to the mask shop (probably by courier, as this was pre FedEx). Later, masks were made by e-beam which was a raster scan instead of a list of fractured polygons, in a format called MEBES (manufacturing electron beam exposure system). But it was till a tape in the early days. Then, eventually, MEBES files were transferred electronically. But even though it is decades since anyone truly released a design to manufacturing with a tape, we still say "tapeout".
Old standards live for a long time. Imagine the guy (I know, but look at the date, it was a man for sure) in 1956 who designed the cigar lighter in automobiles. He would be amazed that there is at least one in every car 60 years later, and even some planes. But he would be even more surprised that computers thousands of times more powerful than the ones he'd maybe seen on TV, that filled large rooms, would fit in your pocket...but still use his design to recharge the battery. It is actually a horrible standard for that though, since the plug has a pin in the middle that is trying to push the plug out of the outlet and it is only held in by friction.
But my favorite old standard is this: why do we get on planes from the left hand side? The reason is that the first planes were sea-planes, so of course you got on the left side. Because that is the side you get onto ships, the "port" side that faced the quay when the ship was in port and where the gangplank would go. The other side, the right side, is called the "starboard" side. That is a corruption of "steer board", the steering oar that was used before the invention of the rudder. This was put on the right side since that is most natural for a right-handed steersman. The port side was put against the quay so that the ship could be maneuvered into position without the steering oar being blocked.
So 2000 years after the invention of the stern-post rudder, we get onto planes from the left so that the steering oar isn't obstructed by the airline terminal. Now that's an old standard.