The gas is about $4 today. A gallon of gas will buy you at least 30 miles of travel. A bus fare is $2 for a single ride. If you are traveling less than 15 miles, it is not cost-effective. If your trip is longer you need to pack your lunch, and on some routes you also should write a will.
74% use personal cars because cars are better. The 26% are either too young or too old to drive; some are too poor to own even a junk car, or have nowhere to go. A car is an attribute of a reasonably successful person. It's your personal space that protects you from other people who you may not want to interact with. Do you think Bernhard Goetz would see a car as a worthy investment, given all the troubles that he picked up on a subway train? Mass transit is a major crime area; things are always happening on buses, on trains, and near them.
I took a 150 mile bus trip once. Sure it only cost me $17 but it took nearly 7 hours vs the 3 hours it would have taken me to drive for about the same cost. The bus also dropped me off nearly 20 miles from my final destination despite passing within a few hundred yards of where I was going.