First, they're not likely to get someone starting a competing, open infrastructure with Cable because regulations in place for ages probably makes Comcast your only CABLE provider. So do you want that really high speed access through your cable provider? Well, nope...can't have that.
Second, depending where you're at your other options may be limited as well. Many places you're stuck with one phone company and can go with the generally slower DSL service. You may or may not have access to Satelite internet, largely depending on your location. You could go with a cell company if you have good reception, though that will potentially give you much slower connections or a severely limited amount of usage.
Third, it fails to understand that ultimately even if you COULD start up small competing companies, they'd still likely be piggybacking off the Teir 1 company, Comcast's, backbone in the area and thus still subject to the same throttling issues as if you were directly working with Comcast. Mind you, backbone that we the people largely subsidized and paid for.