This is hinted at in other answers, but it really is the key. There were two ways that a video terminal (or a Teletype, for that matter) could be used:
A terminal could be a computer. The Datapoint 2200 is one of the earliest examples. At its heart it was a terminal. But plenty of things could be added, including a hard disk drive, floppy disk drive, network card and additional RAM, so that it could indeed be used as a standalone computer. The 2200 used one processor, which was the basic design for the Intel 8008, to run everything.
A terminal could also include a computer in the same box. An example of that is the Heath H-89/Zenith Z-89 which used a Z-80-based video terminal as the housing for a full CP/M (and other OS) computer. For a non-technical user, it was one magic box. For a technical user it was a computer and a terminal combined together.
Many other computers, commonly running CP/M but not exclusively, had one box (or sometimes more than one) with a computer and a terminal, such as a VT-100, connected via a serial port.
In all of the above cases, this was one user = one computer.
This was the primary use case for VT-100 terminals and, I would argue (but don't have the stats to prove it) for most serial terminals. A single computer, from PDP to VAX to various microcomputers (8080 or Z-80 with MP/M, 8086 and above with MP/M-86, Concurrent DOS, PC/MOS and other operating systems, plus numerous other systems from Data General, Alpha Micro and others) could connect to anywhere from 2 to dozens of serial terminals. While a multi-user system could be used as a bunch of nominally separate user spaces, sharing the cost of CPU/hard disk/etc., the big advantage of a multi-user system was (and still is today, though more commonly in the form of a networked system) sharing files (instead of having to physically transmit or copy on a floppy disk, etc.) and sharing applications (e.g., accounting, inventory tracking, etc. all using the same data).
If you are DEC then you want to (a) sell more PDP and VAX systems with lots of terminals along with the systems and (b) sell lots of terminals to competing multi-user system vendors by building a high-quality, standard setting terminal such as the VT-100.
Many large multi-user system vendors sold either their own terminal or a rebadged version of someone else's terminal. WYSE eventually made some of their own computers, but their main product for many years was terminals that could be connected to other vendor's computers and emulate the native terminals, including DEC terminals.