why wouldn't they?
After 2017 I'll never use US-east-1 again. Hell... I should have learned that particular lesson in 2011 but it took two catastrophic failures for me to figure it out.
There are numerous threads here on HN covering the topic "why does US-east-1 suck so hard."
https://news.ycombinator.com/item?id=13756082 is just one example.
I kind of doubt that they’re getting efficient use of hardware Amazon could be using but isn’t, since if someone allocates it it’s not available for Amazon anymore.