There are a number of reasons but the biggest is, they simply can‘t.

There is no rejoining, you would have to apply for membership again. That process takes years even if every member is friendly towards you, which in case of the UK is not the case.

A rejoining is also currently not beneficial for the EU. So they have little incentive to allow it. The drain of firms towards the mainland is just starting and the EU needs that process completed and some time extra to flourish before they can think about talks about rejoining.

Another problem is the UK itself. While heavily divided, an absolute minority of voters are politically in line with where the EU is currently and where the EU is heading. So rejoining would be a hard sell in the UK.

The majority of people in several states seem to think this is the way to go. Otherwise this degenerates wouldn‘t be able to win elections.

I rly don‘t see it that way. The main reason they went for ActivityPub was to fuck Twitter. They just want to bind users long term to Threads and the combination WordPress, thumbler and meta is a very strong argument. I don‘t think they care so much about the data of the small userbase. The care about potential ways to monetise NSFW content without alarming the advertisers. And they care about AP because that can make them even bigger then they are now.