[James_K]

This definitely seems like the sort of change that would break things in unexpected ways. I think the main reason is probably that a lot of websites used tables to do grid layouts before CSS grid was a thing, so having sticky headings would break this. It's really a huge failure of the semantics of HTML that a table is viewed as "an arbitrary grid of elements" rather than "a way to display tabular data".

[deprecative]

Back when we'd use HTML tables for the entire layout. CSS wasn't a big thing and it couldn't do what you'd want. Even today plenty of emails are done via table hacking. I think your overall point is correct but the origin of the behavior, both human and web layout, are older than that.