The reason it’s popular is because most people don’t understand the economics at all. Someone can go door-to-door in a neighborhood offering to clean solar panels and making huge claims about improving efficiency so much that you can’t afford not to do it.
Homeowners who invested $30K in solar panels think of a $300-600 charge as an easy choice to keep their investment producing money.
My parents had someone going around their neighborhood trying to charge $50/panel for cleaning. They didn’t even climb a ladder. They sprayed washing solution from the ground. Most neighbors declined, but you could see the guy spraying down a couple roofs.
Easy way to clear $1000+ per day for knocking on some doors and pointing a hose at a couple rooftops, at least until the public figures out the scam.