The point being that for families traveling from afar (tourists) public transport to Brighton is expensive and prohibitive so provision needs to be made for motorists. Sometimes a car is the only viable choice.
TBH I'm surprised your journey is so quick.
I'm not surprised the train is so expensive.
Even with the speed of your car journey, it should be considerably quicker.
The train journey should be considerably cheaper.
Your 'final mile' journey into the centre of city X (in this case Brighton) should be equally smooth and cheap, just not by car.