Because even if cars ran on pixie dust and emitted nothing but unicorn farts, they would still be catastrophic because of the way we have to ruin our cities to make space for them. Not only is car-dependent, low-density zoning the root cause of all sorts of problems, from the housing crisis, to obesity, to microplastics (most of which come from car tires), to declining social capital due to lack of “third places”, the consequences of car-dependency include huge carbon emissions beyond just the cars themselves:
Producing the concrete to build all those parking lots and widen all those roads is itself a huge and unnecessary source of emissions.
The energy needed to heat and cool single-family homes is hugely greater than that needed for dense housing, since all six sides are exposed to the environment instead of the also-conditioned spaces of neighboring units.
All the extra paved surface in car-dependent areas contributes to the urban heat island effect, exacerbating the problem mentioned in the previous point.