As per my knowledge of Scooby Doo episodes, there are a few episodes where Scooby and Shaggy said they don't want to lead the monster to the trap. So they have to choose someone else, but no other member agreed. They again used Scooby snacks to bribe Scooby and Shaggy.
So to me the reasons are-
- Nobody else from the team wants to do this
- Scooby and Shaggy can be easily bribed with Scooby Snacks
- They have more expertise in leading monsters to traps
If you analyze this, then you will see there is a proper task distribution among Scooby's gang as per their skills. Daphne Blake can never be a choice because her focus is on makeup; similarly the other characters also have expertise in their own, separate work.
I think their are some exceptional episode but don't have link to them.