Because they actually know how to implement fault tolerant scale on AWS. Most companies don't have simian army randomly wrecking infrastructure. Many companies

Because they actually know how to implement fault tolerant scale on AWS. Most companies don't have simian army randomly wrecking infrastructure. Many companies/engineers think they can spin up a few (or few hundred) ec2 instances and think they have solved resiliency. AWS is a tool, not a bullet made of silver.