It's funding. If the transportation agency had money in their budget, they could just build it as far as I'm aware. Most transportation agencies don't have enough slush money to randomly build a bridge. Taxes are unpopular, people don't want to spend that much. It also means the cost is localized, so the tax increase would be higher since it's over fewer people.

The way I rationalize it is that despite the fact that it might be a bridge in Nowhere, West Virginia, it will likely have wide-reaching impacts. More trucking routes helps almost everything. If I'm a visitor, I'm going to use the bridge, so I should pay some part of that.