Because players become available late in the window through a domino effect of other moves either being completed or clubs realising they need to shift players out. Opportunities pop up at the last minute that weren't there previously and you can get bargains if you're looking.
It probably is something along these lines, but it still just seems so messy to me. In an industry of such attention to marginal gains, so much money and so much at stake it's incredible that clubs can't figure out a better way of handling the transfer window.
It sort of makes me wonder, what would happen if there were no transfer windows and you could just sign players any time. Would anyone ever actually move without the pressure of a looming deadline? Or would it just be a constant cycle of everyone waiting for someone else to twitch?