It turns out it's easier to hire people who specialize in scalability, reliability, debugging of distributed complex systems, cost ops, efficiency gains, etc. than it is to instantly ensure every single engineer that works there is an expert in all those things, without anybody on staff who is an expert in them. Much like database skills, IT security skills, etc.

It's still something everyone can theoretically learn, of course, and someone who works in that role is most effective by instilling those disciplines in others as they go, and making everyone successful.