There are ways to implement a vehicle miles traveled tax that don't have scary privacy issues, but no one seems to consider the cost of enforcement, especially if the tax is at the federal level. At least states could use sticks like revoking licenses for people who don't pay, but it's not clear how the feds could do that given that states issue licenses.
It isn't that it can't be enforced, it's just that you're going from a tax levied on a relatively small number of retail businesses (gas stations) to a tax levied on every automobile owner. Enforcement would be complex and costly. Unless, of course, you require everyone to have an autopay credit card set up to your GPS surveillance device, but... not everyone has or can get a credit card.