If they had sent Bautista a script he might have accidently left it on the bus or something, and since Marvel needs to be sure that nothing about the movie leaks, they probably have the script under lock and key someplace.
It’s common for movies these days to only distribute copies of the script as needed, often only the part necessary for the scenes that are being filmed.
To be clear, it is apparent from Dave Bautista’s tweet, that includes video of Elizabeth Olson and [Jeremy Renner][2] admitting they don’t know anything either, that there, in fact, is a script for Avengers: Infinity War, it’s simply that the man who plays Drax has yet to see it.