Because some people can't "afford" the price of the unsubsidized Nexus 5 next to a "$199" (on contract) S5. Some people like better cameras, some people prefer the S5 hardware, etc etc.
Maybe it's not a matter of "affording" the same way most people cannot afford to pay for a house on the spot, but I can understand that some people prefer to pay ultimately more money, but a much smaller sum over time.
If I wanted to, in the US, could I get a contract with a lower value because I'm not buying this subsidy? If so, wouldn't the right move would be to do so and buy an unlocked phone? I'm guessing AT&T/Verizon's interest is far larger than banks', leading to a better deal, even if you don't have the money right now.
On the other hand, locking my phone is the wrong way to enforce anything. It's my device to do what I want with.
ATT and Verizon to my knowledge do not offer equal quality service as their contract plans at lower prices for BYOD customers.