because they are the best for the job and ithink thats one of the concepts of the story,that shaggy and scooby are scaredy মার্জার 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 দ্বারা ফ্রেড 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. ডবলুডবলুই wrestle-mania. Did আপনি see how many ছবি they had?! Those had to be from just one month! They don't appreciate Shaggy and Scooby.