Wasn't it because they hit the scale where managing their own data store, renting colo space, etc. was cheaper than AWS? I've heard first-hand from some large players that self-hosting is cheaper. One is still-hosted, but one is on the cloud as a strategic bet.
Cutting out the middleman and their profit margins is almost always going to be cheaper if you have the requisite scale requirements. Depending on what your workloads are (particularly non-bandwidth intensive), that scale can be pretty damn large for compute infra nowadays.
IMO, I think you're undermining the multiple years of programming/sys-admin experience it would require to self-host + scale on your own. It's much more expensive when experience is minimized, as a function of time.
