Again, as has been stated several times, they don't necessarily even buy scripts because they intend to produce them; they can do it to prevent other studios for going in on it (otherwise you end up with stuff like the Jungle Book situation) or with the full intent that they'll only use elements of it; something as simple as literally "well I really like this supporting character here, we could do something similar" might merit buying the script, because if you have something similar and you didn't pay for it they'd be within their rights to come after you. One thing the Writers Guild is actually very good at is making sure screenwriters get their due.
This is an account deliberately given with no background into how films get made to get more exposure and outrage.