Because they might not want to do the movie and a contract is no sure bet. If you got a good agent or you have enough leverage it's possible to get out of those things
I imagine they singed for three "solo" films--much like RDJ, Evans, et al. Further, contracts are not easy to "get out of", and its is doubtful any of the GOTG actors would dare hint that they might fight the terms of their contract(s) as it would likely not go anywhere good for the actors.
Also if they are relucant to do the movie that is something any future director will know about and might not want to get involved if he or she thinks the cast is going to be hard to work with.
One, Disney call the shots and pulls the strings, not the actors. They signed their name, and now they have to dance, despite their disgusting support of Gunn. Do you really think any of the GOTG actors is going to risk their careers being a problem / refusing to do their job all because of their reprehensible whining over Gunn not being brought back to this series?
Oh, I see the screengrab of Bautista's statement. "Legally obligated" is all you need to know. In other words...contract. He will jump whenever Disney demands it.