It's about queueing work, not running all these queries at the same time. You can run pgbouncer or you can have a pool on your backend. Having more connections won't make it go faster, so that really seems like a low-priority thing for postgres to me. Even if you integrated pooling into postgres the overhead of auth would be still taking time for small queries anyway.
that's too simplistic.
there are many reasons to need something like pgbouncer.
1) not all workloads lend themselves to backend pools
2) not all backends can afford pooling
3) you don't always control your backend
4) you might have a lot of different backends connecting to the database. it's much simpler to just tell them to connect to pgbouncer.
5) you don't want to trust backend team to not bring postgresql down with many simultaneolus connections.
6) backend pools often misbehave
people don't wake up one day and just add another cog to their infrastructure randomly.