Because public transport is crap
I travel about once a month by train for work, and there has been one journey in the last 12 months that hadn't been impacted by strikes, overtime bans, cancellations, landslip etc etc etc
Never mind if you are less able bodied (I'm not implying the family members are just speaking from my own experience), the last minute platform changes that don't leave enough time to get to the new platform and can sometimes be badly announced for the hard of hearing, Euston only releasing platform numbers a short time before the train arrives so you have to rush, lifts out of service, a lack of staff for wheelchair ramps and them forgetting you need someone when you arrive at your destination, etc etc etc
Oh and trains where you can't reserve a specific seat so it's pot luck and depends on which station you get on at as to whether you even get to sit down, not much fun on a long journey or with a disability.
I've travelled through 14 countries of Europe by train (and counting) and I can always rely on the fact its the British trains that will muck my journey up
And whilst we do have planes not everyone lives near an airport to make it worthwhile. My journey to one of the offices I go to is 5 hours away by train. An hour and a half by plane but then I am a 2 hour train journey away from the airport so its not really worth it.