It's called road tax because you need to pay it before you can take your vehicle on the road. You can own a car and keep it in your garage and you don't have to pay any tax on it - it's not a car tax, it's a road tax.

It's an error to assume that people call taxes by what they pay for, the common name of a tax is what makes you liable for it. Income tax is paid when you earn income, insurance tax is paid when you take out an insurance policy, road tax is charged if you want to take your vehicle on the roads.

'plenty of vehicles' is also just bicycles and carts AFAIK, any other vehicle requires registration and tax (even if the rate is £0). In any case, it doesn't matter that not all road users pay road tax, we still call income tax as such even though not all earners pay it!

As an aside, I don't own a car and in fact prefer to cycle; I simply object to a factually incorrect and nonsensical prescriptivist campaign against how normal people speak.