Earlier quoted context omitted.
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.
Even if this is not the case, applications alone are getting larger all the time. 256G is ridiculous as a base config.