If an american manufacturer wants to get into F1, they would buy their way into an existing team, not start one from scratch. It would be an "american" team based in that part of Britain where all the F1 teams are. You don't build an F1 car out of any old road car factory. The facilities required are rather specialized. And if at all possible, you wouldn't base it in the US. Logistical nightmare considering the majority of the races are still in Europe.
Second, I suspect most american car makers would much rather run in something like ALMS/USCR/GrandAm because in those series, you can race what you sell (or at least, something that looks and sounds like what you sell). That's a much better marketing opportunity for them, and it costs a hell of a lot less than an F1 team does.