A lot of AWS services (especially SageMaker) aren’t very good in customer experience. People buy them for nominal capabilities and AWS core bread and butter — short-term and long-term reliability.
Most of these startups (AI and others) have to offer a compelling product before even being notable.
Besides, AWS top level doesn’t care if you use sagemaker or not. There’s a premium but if you’re still using EC2 via another startup, they’re still capturing lions share of value.