Why do membership plugins use WordPress’s native user system

I looked into a few membership plugins (Ultimate Member and Members) and they all used WordPress’s user system. I was expecting them to be more standalone and have their own table with users, but instead they just create a new role in the WordPress’s user system, then give that role to the new users that register via the website.

Why do they do that? It doesn’t seem to make any sense, since I have my WordPress admins/back-end people and my regular users in the same place, even though they are completely different.

By “WordPress user system”, I mean the admin/author/contributor thing.