Hmm. Maybe I'm just a mediocre maintenance dev but a decent tech lead. I went back to being a dev afterwards, and I was shocked by how much harder I found it.

I'll say one mistake is to have a tech lead that spends most of their time coding. I don't know how you can do everything else effectively if the company still relies on you to deliver code.

A possible explanation is that leading a team effectively isn't necessarily more difficult, but rather that it might be less pleasant and/or less financially advantageous than being either an individual contributor or an ineffective team leader.