I think E2E tests can be a heavy lift for many developers. For example, I just now have gotten our Okta login working consistently in Cypress after months of trying. For now. There's no guarantee the next release of Chrome, Node, or something random I can't yet predict will break it again. It's pretty risky to put a lot of effort into tests that will only run if the flakiest part doesn't flake.

Next, it's far harder to figure out how to both exercise the code and verify it did what it was supposed to than it is to just write the code. This is ironic, since test engineers don't make as much as the production engineers they have to be better than. But since most devs are under pressure to do the production work in a short time-frame, they don't feel like they have time to get into the weeds of what are essentially several other languages and a completely different way of thinking.

And, finally, there's an art to building the legos needed to make it quick and easy to write tests, and if you don't have those the only people who will write tests are the ones who you basically couldn't stop from writing tests if you tried 🙋‍♀️. And that in itself is a skill on top of the ones necessary to just do any testing at all.