They absolutely could, and should, so why aren't they?
The answer: Despite working together , these are 3 separate owners, with 3 separate parcels, who cooperated only enough to place roads through their lands in order to unlock their entitlement.
With 3 discrete owners, they each have a parkland obligation. Consolidating those would mean forsaking at least one tower on somebody's site. To do that, the other owners would have to provide compensation, plus, the City would have to formally credit each of the non-providing parcels with credit for the provision on the third site.
You would also have to mass the site completely differently with one large park, trying to provide it proper access to sun. I can pretty much tell you where it would go.....and well..... LOL
****
Yes, you're right, that's no way to plan, in fact, it's idiotic.