Big corp financing is weird, and IT is often its own cost center responsible for its own budgets. This means that every time my department asks IT to do something for us they send us a bill. And that bill is always several times more than going to an external provider. At a place I worked, hosting in-house would for example cost a minimum $150/server/month for something with the same spec/performance that would cost $20/month on AWS. It could also take up to 10 days to get that (virtual) server set up. As a project owner and someone responsible for my project/department budget using an external cloud over in-house hosting makes rational, economic sense

Building up an IT infrastructure that can provide services at roughly the same quality and cost as AWS takes a large investment in talent and infrastructure. And many companies don't see the value in making that huge upfront investment for something they might not need. Better to start with AWS, and once you get to the point where you can lower your costs by 50-70%, then start looking at moving in house.