Because I would. In the UK you can drive at 17, just pass your test and you never have to worry about it again for the rest of your life.
I think people should be tested every 5 years. Driving is not a right, it's a privilege, and if you want to continue receiving this privilege you should show you're fit and able. If you're hurling a box of metal around roads up to speeds of 70mph (and over, despite what the law says) you should demonstrate more than once in your life that you're able to do it safely.
FWIW, I'd also introduce regular testing for cyclists.