Where else would they go?
Bluesky explicitly does not want accounts like Ars Technica on it. Threads might as well be Facebook for how effective it is at letting anything you post there be seen. It should be clear at this point that Mastodon is not a useful platform for most people.
I'm sure a lot of people would love to abandon Twitter, but other than abandoning the concept of social media entirely, there's nowhere to go.
Perhaps, but then that is the new reality, it's not like a platform with wide and easy reach existing is an inalienable right of humanity. I'm sure the money on the line is non-zero, but then the cost of staying is going up, associating with this crap while pretending it's still OK.
Clinging to the fast sinking ship to scrape every possible coin out of it is not a great look for any organization.
Something, possibly ActivityPub based that is a better fit for wider consumption, will surely come. Those affected, with the means, can invest in making it happen, as some orgs/companies are already trying to do - like making Mastodon itself better.
It's probably easy to armchair this and I don't see how joined at the hip with Twitter some entities are/were, but then again, the reality has changed, and the bird is truly gone, even if there isn't a replacement currently.
Edit: seeing hestermofet's numbers though, yeah. There's no excuse.