The thing that frustrates me is it’s possible to know how to do both. I have worked with multiple people who are quite proficient in both areas.

Cloud has definite advantages in some circumstances, but so does self-hosting; moreover, understanding the latter makes the former much, much easier to reason about. It’s silly to limit your career options.

Being good at both is twice the work, because even if some concepts translate well, IME people won't hire someone based on that. "Oh you have experience with deploying RabbitMQ but not AWS SQS? Sorry, we're looking for someone more qualified."

That's a great filter for places I don't want to work at, then.