Actually, I imagine that if you could go multi-regional then your self-managed solution may be directly competitive in terms of uptime. The idea that in-house can't be multi-regional is a bit old fashioned in 2019.

For several reasons, most notably: staff, build quality, standards, knowledge of building extremely reliable datacenters. Most of the people who are the most knowledgeable about datacenters also happen to be working for cloud vendors. On the top of that: software. Writing reliable software at scale is a challenge.

Multi/hybrid means you use both self managed and AWS datacenters.