Why No Wireless Mesh Connection Sharing?

OK, of course what I am talking about is a violation of the T&C of every ISP I have ever used, but really...

What am I talking about? One of the greatest, yet least used, ideas for wireless networking is the idea of the wireless mesh network. The idea is that if my WiFi device can talk to your WiFi device they can create a network that includes those two devices. If any one of those devices has an Internet connection, then all of them can share the connection and all access the Internet.

OK, yeah, you've seen that, done that, know someone who does that...

I have a wireless router that is always on and connected to a 10 Mbps Internet connection provided by the local cable company. From my living room my laptop can always see at least 3 other wireless routers and in the evening it can see as many as 12 different wireless networks all running in my neighbors homes. Each of them has their own multimegabit Internet connection. So, why can we all share those connections and load balance across all those connections?

Seriously, why can't I allocate some off my bandwidth to share with my neighbors? I'm not talking about letting my neighbor into my network. I'm just saying that I would like to let my neighbor send and receive some packets through my router. Those packets would not ever be allowed to be routed into my network, just sent along to the Internet through my connection. By sharing the load we would all see smoother faster Internet access.

Yes, I do understand that there are a lot of problems with the idea. Just getting the addresses all sorted out would be a tough problem to solve. But, wouldn't it be nice?