The adhesive property of dried out poop is amazing. Even on something as slick as a porcelain toilet bowl. Simply wetting it doesn't remove much at all. It takes a pretty aggressive scrub to get the stuff off. I would suppose some sort of enzymatic action would have to take place to remove the stuff when there's no way to scrub or blast it off.
I start a trip with 3 gallons of water in the black tank and add at least a half gallon per flush of solids. Then I add 2 packets of dishwashing detergent to the tank at the start of the trip as well. Seeing as a dishwasher is able to remove stuck-on stuff with just water pressure, it's my theory that it will help in a black tank too. Gray tank for that matter as well, so I add 2 packets there too.