I thought about how to design pizza delivery drones and came to the conclusion that perhaps the best approach would be to make a very heavy armored tank, with a pizza oven built in.
Why don't people do it? Generally, very few people commit crimes in the first place. Especially if you make it difficult, and extra-especially if they are backed up by rule of law in your country.
If they could do it all in the truck and cut out the kitchen it would be even better. I guess that's essentially just a food truck that does deliveries though - which might not be a bad idea either.