TCP hole punching is goofy and unreliable, last I checked. You have to do some arcane ritual of having both peers start a three-way handshake to each others’s public endpoints simultaneously, relying on NATs to accept inbound SYN packets if they match the outgoing SYN. And nobody’s NAT devices implement simultaneous-open the same way, so all your connections just fail.

Naturally this leads to slapping even more arcane fixes on top of that, like NAT port assignment oracles to adversarial interoperate with different port allocation strategies (random, sequential, single, etc.) by analyzing patterns in previous port assignments. Networking sucks.