I assume they want fstab integration because the goal is to have everything as fully declarative, and also because mount units have a much more explicit, spelled out syntax.

It's also far easier to edit with automated tools than fstab, since it's separate files and there's no ad hoc text parsing to change one line.

I am guessing they didn't do it in a more modular way because the target audience doesn't really have any intent of swapping out many pieces of the OS, and it would be extra work to implement?

If you want to do stuff the devs intended, in the way they intended, it's fantastic, but it seems to disappoint people who need or want to customize.