Plus, as mentioned, you get a vast menu of services to choose from, all of which are managed. Just in terms of databases alone, you could go with RDS (raw MySql/PG), Aurora, Dynamo, Elasticache/Redis, DocumentDB (Mongo) and more. Plus managed backups, failover, multi-AZ, security, and connectivity built-in.
If your team is filled with devops engineers, then, sure, go with the non-AWS route. But it's a lifesaver for agile companies who can't afford a lot of infrastructure work and need easy levers to scale, and can afford a modest markup.