Because it's significantly cheaper and diversifies your presence across multiple providers.

It's a good idea to split origin and CDN, this way requests can be served from cache while origin is down or overloaded.

Plus I'd challenge the suggestions that S3 and CDN are a tiny portion of the infrastructure. For a lot of sites it's a significant chunk, things remaining wouldn't be many, maybe compute, DNS, and CI.

It might be only part of your infrastructure but on many projects it’s a significant fraction of your total expenditure and storage egress is one of the things limiting use of external services. Having a cheap data access option makes it easy to reconsider those choices when you don’t have to factor egress into the cost of trying an alternative.

> Double paying for storage is still going to be cheaper than egress fees overall.

because of this?

There are many applications that have no dependency on AWS at all, other than S3. This is one of the reasons projects like Minio exist.

If you don't care and just need some blob storage, this could be great.

Zero egress costs for assets is a very strong reason for people to move. For public assets, it's a complete no-brainer.