The simple fact is that geometry hates cars. Cars are an amazing way to travel, and for most places in America, nearly the only way to travel. But cars take up a lot of space (most of which is parking). Most places, that's a non-issue: We've mandated a 30-year supply of both roads and parking. But for anyplace that existed prior to those mandates (any main street, downtown or CBD more than 50 years old), those mandates didn't exist, and adding either lanes or parking requires demolishing buildings.
We all understand that adding a lane for a block doesn't do anything. If it's going to do any good, it's got to go on for a good long while. So adding a lane for five miles requires acquiring five miles worth of front yards. And fighting with a few thousand property owners. And that's very difficult to do these days--lawsuits and lawsuit-induced delays are an inevitability.
I've written previously about the extraordinary racism of urban highways, but here is the nut: the land was never cheap, the marginalized people it was taken from just lacked the legal rights to effectively contest it, and get a fair price for what was being taken. Where they did (DC) you'll find a near-complete absence of urban highways.
Cars require a lot of land. They aren't making any more land. With fixed supply, as the demand rises, so does the price. So every bit of land devoted to cars is expensive and mandating that land be devoted to cars is just imposing a cost that, given free choice, people would not choose to pay.