Owls have short legs because their main means of hunting is in the trees.
Owls hunt mostly by night, and flying through the tree canopy requires shorter legs because, while it’s possible to hop or flap to a low-hanging branch, they can’t climb up it like other birds. Furthermore, with no prey visible on ground level at night due to nocturnal animals’ natural wary nature, this would require unnecessarily long leg muscles that would use up valuable energy supplies before any food was captured.