The problem is that it would cost more. In fact, you can already just buy a phone (unlocked) at many online places. The reason it isn't popular is because it costs significantly more - to the extent that even most geeks go with the cheap, locked stuff.
Now, would it make the monthly plans cheaper? Probably, but not by as much as the additional money you'd spend on the phone. Why? Adding a customer, even if they bring their own equipment, costs money. Billing needs to be set up, often it takes time from a rep, new customers have a ton more questions, etc.
So, if you'd be willing to pay an extra $50-100 (and convince 60% of other people that's a better way too), then the industry might change. Until then, we're stuck, but we have cheaper prices.
In the netherlands you can get a monthly plan for 50% of the original price if choose to go "sim-only" i.e. without a new phone. Depending on the amount of minutes you buy and the phone you'd want this can be significantly cheaper.