I'm amazed by how much the list of regions has gotten over the years. Most companies I've worked for have only used at most 2 regions for EC2, more for S3. Aside from Cloudfront, are there use cases you've seen for having compute in every region?
In fact, it's critical enough in many applications that clients may want to access the services in the same cloud they use /as well as/ in the same region (or potentially in their own data center.) So that means designing the systems to deploy on any cloud provider.
While this does limit the number of niceties you can use from a single provider, it allows for some pretty amazing flexibility.
You're still restricted by physics with edge computing. If you need to make a call back into us-east-1 to get data out of a data store, it doesn't matter that there's an edge CDN and API layer.
Outside of that, there's compliance reasons too. Partitioning your data by region because storing customer information in say the EU is different than the US. More likely CN is much different than the rest of the world if you do business there.
Those services don't want to be sent intercontinental because it's just an enormous amount of data, and they will incur back-haul charges. Same is true for customer multimedia services.
The other reason is data governance/sovereignty use cases. With a bit of ultra high latency on the side.
My company stores user data and deploys to most AWS regions. It's a big win for some companies to be able to have their data stay in a given area (esp the EU, but I've also heard Canada mentioned).