If you make more space for cars, and thus reduce congestion, the improved traffic conditions will attract more drivers until the congestion reaches some boundary threshold again where the majority of the remaining drivers are no longer willing to spend their time on the route.

The problem is that the demand is virtually indefinite for a nominally free supply.

The only cost involved is time―time that is wasted in congestion―which limits the amount of drivers willing to use the route during congested times.

Dynamic tolls would make the cost explicit.

If the cost of using the road at any given time would be high enough that there's barely no congestion on the road, i.e. practically everyone can cruise at the limit, then people could choose to pay for the privilege, postpone their trip to a later/earlier time slot when there's less congestion and the toll would be lower or zero, or omit the trip all together. Obviously, people wouldn't like this because they've used to getting free access, or "free" if you don't count the billions of tax dollars that go into widening and extending this utopia of endless freeway capacity.