Carriers do max out the capacity of many cell towers. Remember that it's you and dozens of other people (at least) concurrently downloading stuff over the same tower. Scarcity is a fact of life, and the bandwidth has to be allocated somehow.

I do wonder how it would work if we did some sort of high-speed dynamic bidding for bandwidth, rather than a constant fee. It could actually be very helpful; it would incentivize carriers to appropriately allocate bandwidth for an area (maximizing profit would coincide nicely with maximizing customer utility) and it would allow customers to directly "feel the pain" of bloated downloads and websites, leading to more market feedback for wasting bandwidth by making 8MB webpages or whatever.

Here's a really cool idea; no SIM card, no network attachment. Your phone uses some technology like Bitcoin micropayment channels (high-speed off-network secure micro payments, great protocol) to automatically buy bandwidth from whichever regional cell tower is offering you the best deal.