There are other reasons.

Adding, say, a sixth lane doesn't increase capacity as much as adding a 2nd lane, because traffic jams are generally because of interactions. It's very rarely the straight road that has a capacity problem. Adding a sixth lane adds capacity, but also creates more interactions.

Also, car lanes have a shit capacity, which goes down massively when it's busy. Like you said, mass transit is vastly superior, but even a dedicated bus lane would help. In contested traffic, a car lane transports less than a single bus per hour.