Because that is not how wireless communication works. E.g. in LTE the user is scheduled both in frequency and time. The operator has a certain frequency spectrum that gives a certain maximum transmission rate (one user getting all frequencies and time).
The scheduling happens based on (amongst other things) signal to noise ratio and priority of each user and their traffic types (voice, data, etc). The goal of the operator is to maximize the overall capacity of the network. This is beneficial for the user as well.
I think the problem in the US is the lack of competition. Not net neutrality issues. In places where this works well you just change operator if you are not happy with your current one. This way the provider is free to experiment with how to run their network and everyone benefits.