Because I imagine that scurrying back to place themselves staring up with critical concern at my fascia boards/ roof tiles/ roof moss/ windows, ready to greet me with that "thhhhhhhhhhhh" noise with shaking head when I answer the door. Followed by exorbitant quote to fix the "problem."
Why don't they stay on the doorstep? Then they'd get me. Unless they have a giant holdall in which case I'd know it's the hard-luck-story bloke selling disintegrating tea towels at £10 a pop.