Because you don't know how your circumstances will change through your life. Where you might live. What you might work. What public transport will be withdrawn or altered. For how long the drivers that you depend on will still be around, or able to drive you (ill health etc.).
There are plenty of my mother's generation of women, for example, who relied on their husbands to drive then all their life, then their husbands died. Suddenly, they are isolated.
Or what if you drive somewhere and the person who drives you falls ill? How would it not be better in those circumstances to be able to drive?