There are a ton of optimisations that you can do to a webserver. So while "selfhost all the things" might safes you from being that sophisticated when building infrastructure, large organisations might require such infrastructure and hiring a team to do this for you, might be too expensive if you need it only in a few cases.
Therefore going and buying such a service externally, is a legitimate act.
This is my personal microblog. It's filled with my fun, joy and silliness.