The problem is that most americans get their internet over coxial cable owned by the local cable co. Most cable company networks don't have individual loops to unbundle.

You share a loop with the whole neighborhood.

I've heard people claim it is possible, but I'm not really sure how.

They're already running their phone and Internet services on the same infrastructure but different VLANs. It shouldn't require new hardware to add half a dozen more VLANs for the different backhaul providers and apportion bandwidth inside the last-mile network equally between VLANs.

It won't technically be "loop unbundling" but it's not like it's particularly difficult - it's how MVNOs work.