Because no one is willing to change those things. No politician who would be willing to go this far would be voted in because of the intermediate issues this would cause for people. And doing a super slow transition would be too late at this point, especially since we're way past schedule already in regards to our emission models. It even starts with the simple fact that people are simply not willing to get rid of their cars, even if public transport was good and completely free. So you'd be left with enforcing people not to drive, which is obviously also not going to happen for the same reasons.
The #1 priority for reducing climate change (and fixing almost all our problems, from housing affordability to obesity) is zoning reform.
Only in countries like the US, who have a disproportional large portion of transport emissions. But a lot of our emissions in the West simply come from the production of our goods that we buy and give us our comfy lives.