Price and dependency on a single entity.

> Why not just use AWS?

Physical goods vendors that sell through Amazon have found that once they sell something highly in demand, and profitable, Amazon uses all of their internal knowledge of the sales to create their own Amazon Basics version of the same good and then promotes that as a cheaper alternative, capturing most of the profit.

There is absolutely no reason not to believe that AWS will do the same thing and use their knowledge of the workloads and exact hardware requested to compete with you. The fact that they haven't done so yet isn't much evidence against them doing this in the future, since other parts of the same company already follow this approach.