Neat. My new MacBook Pro has 256GB. I'm using ~133GB. That's XCode and the Xcode Beta, the full Office suite, Logic Pro, Firefox, Chrome, Safari, IntelliJ, GoLand, Photoshop, Illustrator, a bunch of other Adobe apps, and on and on. When I'm at the desktop I use an external drive for dumping junk to, and for Time Machine backups.
There is a point of diminishing returns, and given that most people don't have a giant Steam library on their Mac, 256GB is more than sufficient for a large number of users. And as many others have said, when you're working on professional data like video and audio it is almost always on an external array.
You are right. It is sufficient for a large number of users. However, the Mac Pro is a high end workstation used by professionals, targeted for applications like high resolution video editing that require insane amounts of storage.
Relatively speaking, storage is cheap. A high performance, 1 TB NVME drive can be had for under $350. This should be the base configuration on a high end workstation...
What benefit does a 1TB SSD have over 256GB for a video workstation that needs to connect to a 100+ TB array for the real work? If your video editing task can get by with even the 4TB maximum build-to-order option, then you're not in the target market.