Sure but hosting content isn't free, so then you have the problem of paying for it. I could imagine a crypto-currency based solution but that is just sooo complicated.
Most of the reason for centralisation is simplification. Have you tried running your own email server lately?
Yes, cryptocurrencies was one way. (I don't think that it's really complicated.)
But on the other hand I don't think that it's really expensive. As in have a RasPi lying around at home that's keeping track of everything when you're not online. That should totally suffice for your own needs. If you have bigger needs or want to support the network (maybe even for a small compensation in whatever form) that's easily scaleable.
Or think of bittorrent: It's incentivised that you run contribute back what you received. That works totally without compensation in cryptocurrencies.
About the simplification I'm not sure either. Have you tried running Gmail lately? (Not as client but as service ^^ I think that it's not quite straight-forward.)
Once you have a proper working p2p network/algorithm/protocol I can imagine that it's easier to run for all parties.
Most of the reason for centralisation is simplification. Have you tried running your own email server lately?