Developers who don't write any tests are often the most productive.
If you look at them in isolation, sure. If you look at the whole team you'll see that the untested code drags everyone else's productivity down because all the other developers can't modify untested code without fully understanding everything it does. They have no tests to give them any confidence that they're not breaking stuff when they change it.
Developers who don't write tests are fine if they're working alone, but they're toxic to team productivity.