That is a good question. They could just be getting really good rates on memory/server storage, and are passing the savings onto us.
Viewing post in How does itch.io host 1 million free games?
Yes I think this could be it, I've since learned that you can get quite cheap storage from places like bunny.net for like $1000 for like 40TB or blackblaze which offeres $6 per TB examples:
https://www.backblaze.com/cloud-storage/pricing
It could be possible to get 100 TB of storage under $10,000 per month. You may also get more discounts for being a customer that uses a lot of storage. It is still fascinating how itch functions.