PostgreSQL created a PostgreSQL user on mac. Is this necessary?
Usually, the Postgres server process is owned by the accompanying system user (not by root or the installing user), so it only has a limited set of rights, making possible attacks somewhat less dangerous.

Additionally, the Postgres system user is normally used to carry the default privileges to initialize db clusters and access newly created databases without password. But you can change all that, of course.

What MySQL does is hardly relevant to these considerations. It is in no way role model in security related affairs.