Note that implicit in the deal is that the carrier provides support. You can take the phone to the store and ask questions, or call them. For faulty hardware they will replace it (warranty services). The carrier will typically also do insurance for those that want it. The consumer has an ongoing relationship with the carrier. It is hardly surprising phones mostly get bought that way.

There is no such relationship with the phone manufacturers or retailers, with the Apple exception, and even they depend on carriers. (You can fairly accurately predict Apple sales by how many carriers carry the devices.)

Note that being available on a carrier doesn't automatically lead to success (eg Blackberry and Nokia are available on about 3 times as many carriers as Apple is). However it is a prerequisite for getting a foot in the door.