because they are the best for the job and ithink thats one of the concepts of the story,that shaggy and scooby are scaredy gatos and they always get picked for the job!!
Shaggy and Scooby are used to lure the monster/villains into their traps because of how fast they run especially when they see them I believe that might have been explained por fred figglehorn in one of the Scooby-Doo Where Are You! Episodes which I forgotten the name of the episode.
I think Shaggy and Scooby shouldn't be bait anymore, here's why. wwe wrestle-mania. Did you see how many fotografias they had?! Those had to be from just one month! They don't appreciate Shaggy and Scooby.