Of course you can switch databases, but in practice it is extremely expensive. That's a big reason why it is pretty rare. Why use a closed source tool and take on that risk when great open source ones exist?
I'm just saying it's not as bad because it's quite a bit easier to switch a database than to switch a game engine.
_presses X to doubt_
in the case of servers: because open source servers literally can't support your scale of business. That's one of the few places where Open Source can never truly succeed: when you need a lot of hardware and the operating costs exceed any income coming in.
By that point it is a lot better to roll your own servers. But that is of course crazy expensive. Even other multibillion dollar corporations choose to leave some server management to places like Amazon/Microsoft.