So they can rely on the (imperfect) bot defense of those services rather than rolling their own, maybe?

And I assume, provide a way of rate limiting free accounts without someone making a thousand of them

I think you mean Discord rather than Facebook, but yeah. Weird thing to gatekeep.

Have you ever added such services yourself?

It's effort.

While annoying for users who don't want to connect their accounts: very fair point. Cool product!

Just came to post the same thing, bummer.