This kind of trick was used by certain ISP's abusing peering relationships - provider A peers at two points (let's say east and west coast of the US). Provider A builds a tunnel between his peering nodes across B's network and B is suddenly unwillingly providing free cross-country bandwidth. Needless to say, this kind of behavior was (and is) highly frowned upon.
Now - as to the use of BGP over VPN tunnels for transit/end users there are a couple of problems:
1.) The routing of the underlying tunnels is completely opaque. Assuming you have two tunnels to two different providers it is possible (...if not likely in the case of consumer grade service) that these tunnels could traverse some of the same intermediate points, which kind of defeats the purpose if HA is the goal in mind.
2.) Supply and demand - Sure, ISP's could offer these kinds of services but realistically who is buying them? If you're at a point where you've secured an ASN and have either gotten PI space (unlikely) or negotiated appropriate route leaking, chances are that you're spending a fair amount on transit service (at a minimum). In contrast, consumers spending $59 USD a month for Internet service aren't likely to provide the revenue to (profitably) offset not only the cost of the hardware to land tons of BGP sessions but also the operational overhead of maintaining and supporting said sessions. There's also a hidden cost in terms of the backhaul of traffic that's even less likely for ISP's to want to pay for.