I love how Linux distributions still pick a mirror "close to you" for speed, but in 2021 downloading from an S3 bucket across an ocean is faster than whatever university donated servers twenty years ago (for large files, like an ISO)
S3 is extremely expensive for this kind of stuff compared to most options, that's why it's not the default. I can see a lot switching to Cloudflare R2 though!
Object storage services are metered and ridiculously expensive for this use case. AWS S3 is particularly expensive but you wouldn't want to use OVH's Object Storage for this either. Makes far more sense to get as many unmetered bandwidth servers as needed and mirror across them.
Linux distributions end up with the approach they use because they usually don't have much money to dedicate to servers and have far higher priorities.
GeoDNS is nicer than the approach used by most of them but that's usually a premium, metered service unless you self-host.
They can't afford hundreds of thousands of dollars per month for S3. Many could afford to pay for their own servers with unmetered bandwidth but getting mirrors for free is infinitely cheaper. Non-profit open source projects rarely have money to burn on the convenience of AWS.