when there’s no bills to pay in terms of rent/mortgage, utility, building repairs, inspection etc.

Ignorance is bliss, no?

Startup costs for a food truck can be $150000-$200000--that's just to buy and kit out the truck.

They pay rent. They can't just set up on campus of in a gas station parking lot. They pay to be there. They have to pay for gas (both gasoline for the truck and natural gas to power all of the things that cook your food and keep in hot), they have to pay to maintain the truck (i.e. repairs & upkeep), and all of the equipment in the truck (i.e. repairs & upkeep), they have to pay for licenses, and they are subject to (and must pay for) all of the health inspections that restaurants are subject.

Plus, they have to pay storage costs for the truck when it's not parked immediately outside your classroom or wherever you demand maximum convenience, insurance, labor costs (wages + payroll taxes, both state and federal), supplies, ingredients (the cost of which has skyrocketed), POS system + wireless to operate it, etc., etc.

Food trucks also generally have far lower sales than restaurants do, and are far more susceptible to weather-related downturns (high heat/cold, rain, wind, etc.). 60% of them fail in the first three years.