Comment by dathinab

- middleware in the networks not forwarding broadcast messages

- depending on device and application type you not even being able to send broadcast/the OS silently dropping them

- firewall blocking incoming TCP/UDP without hole punching

- p2p in games having security implications (unsafe network stacks, game engine etc. allowing RCEs and similar) so you want to make sure only "more trusted" communication can happen, so TLS is needed, but without actually fully secure p2p TLS is not easy, mainly there are issues with establishing trust (you either have to involve some side channel (i.e. a pin, QR code or similar) or pre-established trust.

The biggest thing is still that as a steam game you have a reliably, proven, easy to use "solution" as part of your normal steam integration which you anyway want to use to be able to use the friend invite system which has the drawback of needing internet for local coop which is niche use-case likely not selling any games. Why would a company implement an additional solution and handle all the UX issues of switching between them?