I'm a very senior engineer, I obsessively write tests, I have yet to be in a team where I'm not the biggest test creator, but even I don't test every single line of code I write.

Some portion of my life is scripts that are only run once. It's usually not worth unit testing those.

Another portion is workflow tooling that speeds up how fast the team can work. But writing unit tests for these is generally much harder than writing the original script, and breakage in these scripts is rare.

For me, testing is a way to find errors in my code, and so I prioritize areas which are more likely to have errors. Sometimes I have hundreds of lines of code with only a single integration test, and then a couple of dozen lines of code with more tests than source - because I know from experience that those two dozen lines of code have a lot of edge cases and I need to be sure.