Also I'm just going to post this figure again because it was so vital in refining the way I think about network-y things.
"Centralized" is gmail. "Decentralized" is gmail. "Distributed" is IPFS or other P2P protocols.
The thing is, "decentralized" is really easy. Decentralized is 1980's tech. There's no technological reason to not have most things be decentralized.
Distributed is a lot harder. That's still an active research problem for many domains.