Wasn't there an edict that amazon internal services have to be designed from the ground up to be externalizable. So even internal technology is designed with being customer facing in mind.
So, even without a mandate, the market forces at play in terms of proposing roadmap projects and choosing what to invest in, will naturally tend to favor those solutions with can pull double-duty (solve an immediate problem/pain-point that Amazon already has, and with a clear path/potential to be converted into or evolve into an external sell-able service too).
[1] https://medium.com/slingr/what-year-did-bezos-issue-the-api-...