In my neighborhood in Los Angeles we have a taco truck and pizza truck that cruise the neighborhood about once a week. Started during the pandemic and it's wonderful.
Taco trucks make the taco on the spot. They have a full kitchen inside (compared to a ice cream truck that typically only has a fridge and a soft serve)
Based on how many people upvotes... I'm guessing people never actually interacted with a real taco truck?
Pretty sure you can't have cooking equipment turned on while driving unless you want microwaved tacos...
You either get cold food, or you wait for much longer then required to get a taco and have to spend lots more because they are burning gas to find someone who wants a taco.