I assume you meant PPPoE?
Because it's a point to point link (the PP part of PPPoE 😉 )
With a point to point link there is only one possible other end, so traffic can only possibly ever go to one place. You don't need to specify the gateway because it can only ever be one thing.
With something like Ethernet it's a multiaccess network, meaning multiple hosts can be on it at the same time. It's due to that fact that you need to specify a gateway. There's no way to know which is the right one (unless you use something like DHCP of course).