Locked phones are a legacy of the carrier subsidy sales model that has carried forward into the financing model.
Apple still offers them that way because it's simple for most users, and because the carriers still wield considerable power, as the great majority of new phones are still distributed through carrier channels, not Apple Stores.
If you want to see smoke coming out of someone's ears, try explaining to them how it can affect their ability to use their phones on trips overseas, and mistakenly think that it's a technical, rather than business constraint that goads them into buying overpriced service from their home carrier to use abroad rather than simply swapping in a local tourist SIM while on their trip.
Most people don't want to think about the technical details, or the flexibility they've given up, and just roll over, giving the carriers considerable leverage. They've been conditioned to associate phones and carriers as practically inseparable things, when, as said earlier, it would be a silly notion applied to other services.
It might have had more truth earlier when there was a clear split between xDMA and GSM standards, but many modern phones are now "world" phones not bound by the technical limitations of the past.
