You are assuming everybody wants to go to the city centre. As I explained, people change at the rail station for a variety of destinations. Presumably "you wonder why people drive cars" is referring to the British and specifically the Greater Manchester experience. Buses go straight to the town centre in Greater Manchester so what I wrote doesn't even apply. It may be counter-intuitive to you and probably most people reading this, because they are so used to the British way of doing things, but places which follow the principles I described typically have superior public transport usage compared to Britain.
But again, public transport (especially bus) use is determined by much more than "interchange". Cost, accessibility, availibility, speed, reliability/punctuality (real or perceived), environment, image etc etc. Sorry, but I agree with 6Gman (possibly for the first time!) the vast majority of bus users do so to access necessities like employment, not to jump on a train somewhere. Yes, statistically, this might apply more to GM than elsewhere, but it still generally applies across most conurbations. Maybe, just maybe, somewhere like Merseyside is different because it had a much earlier (as in well before De-reg) established bus/train integration ethos, which goes with the geography that means you virtually have to go through Liverpool (or at least the outskirts of the city centre) to get from any one major town to any other.