sudo has a lot of machinery for representing complex policies which involve partial access to elevated (or just different) permissions, and with more conditions than just a correct password for the requesting user. The kernel itself just sees a binary running as root which may drop some of those permissions before starting another process.

(And this isn't even the most arcane part of linux userland authorization and authentication. PAM is by far the scariest bit, very few people understand it and the underlying architecture is kinda insane)