Imagine how much better for the environment it'd be if your delivery was brought to you via a human-powered bicycle. Or as an in-between: e-bikes and e-mopeds.
Using 2,000lb vehicles for last-mile burrito delivery is a "we've tried nothing and we're all out of ideas" scenario. Delivery robots are an improvement because literally anything is.
God forbid someone would use their own two legs to walk and eat that burrito at the restaurant /s
The extend to which some people get food delivered is absurd. I'm sure there exceptions and reasons and everything, but seriously.
Why are you comparing them to cars, rather than the (e-)bikes used in most cities?
Why are you assuming that a human would be more efficient and better for the environment than an electrically powered robot? It is very inefficient (approx 25%) to use food as an energy source, and humans are always burning energy. They can't turn off at night or when they are idle. I think it is very likely that the robot would be better for the environment than the person.