> Why though? What's the problem with ipv4?

NAT in general where if you need/want peer-to-peer (P2P) you're suddenly in the realm of STUN/TURN/ICE infrastructure. Depending on your ISP, there's also CG-NAT, in which case (double) hole punching is basically impossible.

If you're 'just' a user, then that may have been dealt with the for you by whatever app/service you're using (e.g., video game companies), but if you're the one having to push out the good/service, it's an entire layer of complexity that has to be dealt with.