That is correct, the median speed, as a rough guess, from the pizza place near my house, to my house, would be 35mph, including the 2 stoplights in the way. Assuming we had proper bike infrastructure(which we don't); you'd be hard pressed to top the speed a car can go, and you would still have to stop frequently at lights, just like a car. And remember, that is the nearest place, not the only. And a small sub note, this area is not flat, at all. The gradient changes are brutal for bikes and they can't sustain a decent constant speed. Well, at least before electric bikes.
I am not defending, in any way, America's horrible car centric infrastructure. It is what we have though, and as a result, bike deliveries aren't an option for the vast majority of America. Of course, when you leave the city, it gets worse.
Because it isn't faster and cheaper in the majority of the US. The nearest Pizza place to me is about 2 miles, the nearest that actually delivers? About 4 miles. And I'm within the city limits of one of the top 20 largest cities in the US. Our population densities are on a completely different scale than the Netherlands. Not saying we have good city designs, but as it is, a bike would a terrible way to deliver food to me.