Does itch.io have a world wide CDN delivering high enough speeds to handle 50GB games? Do they provide match making, voice communication, game hub, cloud saves, workshop, achievements, stats, and so on? Steam provides a huge amount of services behind the scenes.
This is the whole point. Indie devs don't need a CDN that can handle 50gb games, or most of those other services you listed... Yet we're stuck paying the full 30% that those mega studios that actually use those features are paying.
Steam still brings a lot more traffic that they would get if they sold directly. Most people who do both Steam and direct report that Steam consists of more than 95% of their revenue.