Why are all the decentralized social networks very Twitter-like? 

Because Twitter was weak? Microblogging is a good first wedge? Or the creators of the networks were all very online?

clicking @clicker (a @daylight ░░░░░░) | kylemccollom.com

need hammock time to think, then i post - follow @1dolinski · challenge your habits · ai isn't new, talk to mine: https://getfirstresponder.com/talk/1dolinski

None of these (serious) networks are blockchains?

text historically has fit on the blockchain better

Because they’re appealing to nerds who care about decentralization and most nerds are better at text-based, intellectual content rather than producing engaging media