Cant drive in till 18!!!!!?

Hey I'm 17 and i cant drive in till I'm 18. My mom keeps saying no, but she let my sis drive when she turned 16. I don't know why she wont let me, it could be just to save money on insurance or something. We live in a small town and she keeps telling u don't need to drive u can walk or ride your bike around. The funny part is she lets me drive a race car at a local dirt track almost every weekend at least i can drive something i guess.lol And i want to know how many of you out there couldn't drive in till they were 18