Wildly depends. How many people would it take to set up, deploy, maintain, secure, back up something like S3, RDS, SQS similarly redundantly based on top of just raw compute?
For some organisations, it will make sense to internalise and DIY everything. For many others, AWS/equivalent enables them to break down internal silos (instead of having to file a ticket to get storage team to provision storage for your new fancy RabbitMQ cluster, multiplied by 10 for networking, security AV bullshit, etc.) and just consume everything as a service. It's much faster, and in some cases even maybe cheaper.
Cf. Dropbox that started on S3, and a decade afterwards, migrated to self-managed datacenters.
