I think it's because lots of amazons services are in 'winner takes all' markets.
No random online eshop can offer next day delivery across half the world unless they already have a logistics chain of 100,000 truck drivers spread across the world. But Amazon can.
Likewise, no cloud provider has enough data centers to offer multiple separate data centers in the same city, for hundreds of cities around the world. But Amazon does.
Any competitor can't offer amazons level of service until they get to amazon scale... Which they never will.