From Redditor u/rdendi1:
In A Pup Named Scooby-Doo we are introduced to Scooby and the gang as children and learn a little more into the back story of '70s era teens hunting ghost stories for, I don't know, fame? In APNSD, The Gang begins their career as the most gullible and luckiest ghost hunters of all time. We learn the Daphne Blake's family is incredibly wealthy and her father routinely supplies The Gang with equipment to aid them in their paranormal sleuthing. This is where Velma began exploiting her airhead friend and her family. When we get to SDWAY, The Gang has grown up into a group of late-teens who travel the country hunting ghosts and monsters. Many times the teens are simply out enjoying life, being kids who are dirty hippies, living off of Daphne's father's wealth because when they were children they successfully foiled a few cartoon level hijinks as well as falsely accuse a child named ‘The Red Herring’ 29 times (he was the bad guy, once, but he was doing it to help his elderly aunt). The monsters ALWAYS seem to find them. Why?
Because the monsters aren't real. They are always people, yet, the gang never plays off the haunting as a rouse. They have adopted an attitude of ‘monster until proven Mr. Carswell.’ Velma needs there to always be a monster, how else will she milk Daphne for her trust fund? A simple search of public records and Velma was able to identify the next farm to be foreclosed, recent will grants and land transactions, and easily identify a local who would be willing to don a mask and scare some kids to A) get back at the town and B) increase their own savings courtesy of Mr. Blake.
Read the full theory here.