I feel it has a far more strategic plan with substance and history behind it but we see that differently obviously.
To "why not stay on Threads then"?
If Threads implements ActiivtyPub fully and interoperably - then they are a 1st class member of the open social web -- a big IF.
But even so: there is a benefit from moving them off of closed, commercial silos over to Mastodon and others that have things like zero ad tracking, less or no algorithms, far, far better privacy, etc.