It's just a matter of how much they want to invest in what.

In many cases toll roads mean that the government didn't want to/wasn't able to invest in building a road, so they let a private for-profit company do it for "free" (meaning without tax money) and that company then recoups their investment using toll.

Some times toll roads are used to steer traffic. Some cities for example have a city toll that's meant to discourage commuters from using their car to get into the city and instead get them to use public transport.

The first case means the country doesn't raise enough tax, wastes too much tax money or has other priorities than road infrastructure.

The second case is totally valid since it uses tax to discourage unwanted behaviour.

We do have taxes on car ownership, though, at least where I live.