Bandwidth and transfer are not the same thing.
As an ISP you need to provision your links to cope with peak time load. Limiting data transfer reduces customer's link utilisation, so they are less likely to use all their link's bandwidth at the same time.
Peak bandwidth costs because you have to buy transit to that level (minus peering, but unless you're big, peering won't get you that much). Your routers have to be big enough to cope with that traffic level. The links inside your network, getting traffic between your customers' premises and your transit providers/peering points need to cope with that level of traffic. And if those customer connections go through someone else's infrastructure (e.g. a telco's DSLAMs and associated backhaul) you'll be paying for that too, and not at rates you can control by buying better gear to go at the ends of a fibre link.
In New Zealand we've always had data caps, mostly because bits of glass under the ocean cost a fair bit. We used to have them on dial-up connections. Several times ISPs have tried to offer uncapped broadband connections, and they've until the last year or so always ended in disaster.
How have they solved the problem? Mostly clever traffic management -- forcing you to use less of the 15 megabits of DSL last mile link, by slowing down non-interactive traffic during peak load. Ever decreasing bandwidth costs, and a different cost structure for access via the incumbent telco's DSL network, will also have played a part.
Any uncapped connection is going to be sold as rated at a low speed, traffic managed to be slow at peak load, or really expensive. You can buy dedicated, or at least low-contention bandwidth... that's what ISPs and large businesses do. Just don't ask for 15 megabits of low contention internet for the price you pay for a consumer DSL connection: people will laugh at you. 3 years ago I was getting quoted prices of around $1000NZD/mo ($850USD, ish) for a 4Mbps office internet connection. It'll be less now, and obviously as your connection size goes up, the price per megabit goes down. So if you want a connection that can run at its maximum rate, all the time, you can buy one. You just probably can't afford one.
The economics will be different in the US -- wholesale bandwidth will be cheaper. But last miles cost money, routers cost money, backhaul costs money.
And 3G/cellular networks are an even better illustration of this -- there, the last mile is an RF interface shared with a bunch of subscribers. Selling you a 5GB capped connection means you won't torrent incessantly -- at 21Mbps you may be using all (or maybe, just half) of the available bandwidth in your location. Which will make the network slow for everyone else, who will complain that they're paying for 21Mbps and getting 1Mbps or less.