[everythingabili]

I'd say one reason. It's an absolute nightmare for the regular end user to handle relational data in a spreadsheet. I've tried a lot of those below (you missed of AppSheet from Google which kinda falls into this bracket too).

I regularly work with intelligent people (who aren't techies) who by themselves figure out their project needs to be relational all by themselves (knowing nothing about SQL).

I think, and I guess these companies do, that there must be a market for those millions of people who want to manage a few thousand rows of project data but are never going to learn SQL + web development to put a UI on it, but are struggling to keep their data managed "logically" and easily.