Complexity is a great reason to implement something like this in-house. It's probably better to understand (and fully control) the sharding and transaction mech

Complexity is a great reason to implement something like this in-house. It's probably better to understand (and fully control) the sharding and transaction mechanism than to trust a third party with such a core piece of infra. As companies get larger they move further up the stack whether it's sharding techniques, databases, custom orchestration software, their own networking hardware, etc.