I guess they also hire a lot of young people who might not have a lot of practical software engineering experience. They probably have studied a lot of algorith

I guess they also hire a lot of young people who might not have a lot of practical software engineering experience. They probably have studied a lot of algorithms at college though so that is probably the best way to test how good they are at applying things they have learnt. I also guess that when you run the largest online community out there you might run into problems that you can't find the answer to on stack overflow. Even if these are not the majority of problems you face day to day there's an advantage to having staff on-site who have a hope of solving them.