Huh?
You've got buildings on the left side of the screen and the right. Through the middle you've got a clear empty space... but no anchor points to swing from, that's why the cranes were dangling over the middle, it didn't have anything to do with how far his weblines reached.
If his webline attaches to a building on the left or on the right... Spidey's going to lean in that direction... and inevitably, bump into buildings along the way, which'll slow him down, hurt him more, etc.
Marc mentioned that he had the crane scene in mind from pretty early on because he wanted there to be at least one scene where he's swinging clean through the middle like he does in the comics without any obstacles in his way.