Because:
1. Insurance
2. Not all of us can afford to run at 18mpg
3. Price
But you could always bypass all that and get a loan or finance to cover the cost which would lead to all sorts of problems which i wount go in to.
Plus those 90 year olds you see, cash in their pension and are living a freehold life where the only thrill they get comes from the cars (because at 90 taking viagra can be dangerous and sex is no longer a pleasure but a mission and can lead to loss of life)