1. food trucks are subject to lots of regulation and fees too
2. burrito trucks sell their burritos at the market clearing price for a burrito, which is $18 because most of that burrito truck's competition is brick-and-mortar restaurants with expensive rent because of zoning laws.
A lot of the interesting food trucks around me are routinely more expensive than the brick and mortar stores around, often due to the novelty of the cuisine.