Because it's how the major U.S. carriers have operated until last year. Contract-free BYOD options did not become commonplace until after T-Mobile got rid of contracts altogether last spring. A lot of people are still on two-year contracts that predated these new options, so the market impact of these new options is still unknown.
If you look at Apple's website, they presume that customers on 3 out of the 4 major carriers will still buy their phones on contract. The BYOD options are not offered if you buy the device through Apple online. So, there's still a long way to go before the current contract system goes away.
U.S. customers have bought into the whole system of getting a new phone for "free" or for "$199" dating back to the early feature phones, and the monthly fees that might seem outrageously high to customers from overseas, hardly raise an eyebrow in the U.S. because it's what domestic customers are used to. Seeing the actual unsubsidized device cost is still sticker shock to a lot of customers, even though in most cases, a BYOD plan will save money in the long run.
