The thing is that cars are deeply intertwined with other sources of emissions that are much bigger than them, and realistically those other sources can’t be practically dealt with while cars are so prevalent, or at least, dealing with them are much easier in a less car dependent society.

Consider something like oil fractions, when oil is refined you get gasoline, but also fuel oil, diesel, kerosene, bitumen, and others. The production of any one of those is buoyed by the production of the rest, and you can’t do much to control the ratios you’re getting. As long as gas demand is high, all the others will be produced as well, and if they are produced, people will find a use to burn them. Airlines become more fuel efficient or decrease traffic; Previously cost prohibitive uses for kerosene become viable as the price drops due to a consistent supply and a reduced demand from high value airline consumers.

For a serious reduction in oil use, every element of it needs to be reduced in tandem so that the value of no one fraction can keep production high.