Centralisation makes things easy.
If it takes more than 1 minute to onboard to a new service, and especially if you have to overcome any learning barrier (such as what 'instances' are and how to choose one) then the vast majority of people will immediately throw that option out and won't even consider it.
People like bluesky specifically because it gives them something almost identical to what they had before.
Steam has this crazy concept where as a game gets older, you don't have to pay as much for it as when it was new! Pretty wild, I know.