Because a small car won't fit in all the stuff/people they need to carry about sometimes. (Even if when you see them they are driving alone with no luggage, doesn't mean that other days of the week they've not got to fit in 5/6 people and lots of sports equipment.)
Because old cars can end up costing more in the long run if you drive a lot and have lots of repairs/petrol costs.
Because they need something with a powerful engine if they regularly need to tow something. (eg caravan, trailer, horse box.)
Because they regularly need to go off road.
Because they need to drive long distances regularly and want to drive something comfortable/arrive without their back being knackered.
Because they need to be able to rely on the car with no other way to complete tasks they need each day so would rather pay a little more for peace of mind that it'll start every time. (It sounds like you have 3 cars in your household at least so if one didn't start, it wouldn't be the end of the world, someone could give you a lift, and possibly you live somewhere with public transport options.)
Because they enjoy cars and nice ones make them happy. (I see my car as a tool. I recognise other people enjoy them.)