Why would a door be installed without all the hinge screws?
Just some notes:


This is just plain lazy.  Maybe he forgot but forgetting = lazy.
Maybe he ran out of screws.  But these are basically the most common type of door screws so again that goes back to being lazy.
When installing doors it is best practice to get at least one screw per hinge all the way into the actual framing of the wall.  This requires some pretty long screws - 2-2.5" at least.  He may not have had those screws on him.  Again that is not planning well and shouldn't have been left like this, which is lazy.