Why don’t mobile network operators make better use of spare capacity on home Wi-Fi routers to improve their networks?

Many countries are working on this, classical example is Thailand, where the commercial APs are all around in Malls and major congregation points. They have a seamless hand-over policy from Wifi-to-3G and vice versa. It takes a lot of planning and fiber laying in getting buildings up to mark, and at the same time reduces strain on the cell cites that serve heavy population areas.