Because in the examples you can think of, the cities are too far apart to make it worthwhile. Your characterization of "most of the US" is off by a few orders of magnitude. At one time, the two (competing!) rail routes between Chicago and New York were the fastest trains in the world. They are still very competitive today (only about an hour slower than they were 80 years ago), but they are 900 miles apart (by rail; 750 by air)! If you put a TGV-style train on that route, you'd probably beat the current train by about 4 hours (from 18 to 14). It's still completely unacceptable from a travel point of view when you can fly in a little under two hours. The equivalent in Europe would be Madrid -> Milan (best case: 19:30 with a 1 hour layover in Barcelona), Rome -> Berlin (best case: 16:30 with three changes; that's 3 TGV-style trainsets, plus a short hop between them, good connections [one of the connections is a 3 minute window, and I'm sure you'd make it], and relatively straight routing!), or Paris -> Vienna (actually about 100 miles closer) at 12:40 best case.
Look at a map!
The idea of shuttling between city centers and beating airliners in Europe is a MYTH; there are A FEW examples that make SOME sense: London->Paris (but Eurostar is expensive and the security is airline-like, so no chance to hop on with 3 minutes to go before departure; count on an hour at least); Paris to Lyon (but not Nice!); Intra-Germany (but note that Berlin -> Munich, at about 300 air miles, is 6:30 at best on single-train ICE); the three big cities in Spain (Seville <=> Madrid <=> Barcelona: but note that Seville -> Barcelona, at 500 air miles, is 8:19 on the ATR).
In the really excellent cases, it can make a difference: non-stop Paris -> Lyon on the TGV, 245 air miles, is just under 2 hours. It's about the same as Manhattan -> Washington DC, which is only 2:45 on Accela (yes, I think they could do a better job with this). How many other city pairs are there with this kind of closeness that aren't already served on the Northeast Corridor?
The "solution" in Europe -- a handful of fast trains between the major capitols that happen to be close to each other and reachable by the geography -- is implemented in the US by Southwest and Jet Blue.
You complain about BWI-PIT being $400, but try BWI-OAK for $250 on a profitable airline.
/jordan