I would hardly call protocols like [SIP](https://www.ietf.org/rfc/rfc3261.txt) and [RTP](https://www.ietf.org/rfc/rfc3550.txt) "custom". There are many more standard protocols out there than just TCP and UDP. There's even a "better TCP" in the form of [SCTP](https://tools.ietf.org/rfc/rfc4960.txt).
The fact is that TCP was the MVP for reliable streams of data on the 1970s internet, and is woefully inadequate for modern usage, but we're stuck with it (including its oft-disabled parts like Nagle's Algorithm) because it's what's available everywhere.
Game network protocols are pretty easy in comparison with some of the protocols I've encountered in my day job, especially once you've learned the lessons of people like the author of that article.