Because FKs on a relational model.. as an example, deleting a user/account might end up being a task on going through every reference to it, and the references to its references, etc.
This is actually the reason some companies do not delete users/accounts [0].
One of our popular accounts has about 70M worth of rows. I can't imagine how we would go about deleting their data. We rotate out old data each month when it doesn't become needed, but maybe 40M records (4M new each month gets added).
