Edit: For a more US kind of distance look at London to Paris, 2h15m (883 miles as the crow flies) from city centre to city centre.
Paris is not 800 miles, or even 800km.
You're right that a car is unlikely to be the better choice for Manchester-london when ignoring costs (£124 each way on the quiet trains)
To a certain extent you’re right in that it’s all just a question of where you live. But for me recently all the trains I have been taking are in Japan or UK, and if I need to get to somewhere that’s more than a short walk from the central train station there’s been a light rail or underground system for me to transfer onto.
Not to mention that bus fares in the area, in most situations, are cheaper than the metro for same distance covered
Where Metro wins is on capacity. When the trains are full, one track of Metro carries an amount of people that would fill up 10 lanes of freeway. Of course, one major problem is that building mass transit is sold to the public as a way to 'reduce congestion' i.e. "other people will ride the subway so that my freeway will be clear". Due to induced demand from other people shifting their travel to the highway, this is not really truthful advertising.
It's more accurate to say that the mass transit provides an alternative for people who are fed up with traffic jams and want a more predictable way of getting downtown. Buses fail to provide that alternative, because unless the transit agency manages to get a dedicated bus lane along the whole route (it's probably not), they are going to be stuck in the same traffic that an ordinary car is stuck in.