Because I need to isolate myself from a crowd that goes on public transport sick, coughes down my neck and sneezes into my face, doesn't wash properly and doesn't have train manners (NO phone calls, NO food, NO party, NO litter, NO loud conversations, NO music I can hear from your cans). If I don't protect myself, I'll get sick and irritated. Public transport is dirty and littered.
On metros here, there is no 1st class coach I could pay for to get some distance, quiet and respectable company.
So the personal car it is. Unless you fix people, I will use all my privilege to keep myself comfortable.