From my observations, this is a management issue. And as you stated:
"...perhaps a few bugs aren't a problem. Maybe the impact is low enough that training developers to write tests and making the investment isn't valuable enough."
Like continuous integration, devops, etc. a few want a solid product, the many will wait to fix the bugs after the fact. I guess it makes it easy to prioritize which bugs to fix?
The more you are prepared, the less you need it.