It's not just lack of mixed-use zoning; it's lack of dense zoning. On average, something like 75% of the residential land area in US cities are zoned for single-family houses only. Moreover, not all single-family zoning is created equal: for example, in my city most of the neighborhoods are "R4," which means minimum lot sizes of 9000 sq. ft. (roughly 0.2 acres), but the category with the largest lots, "R1," requires a minimum of 2 acres. Even just rezoning the mansions in the fancy neighborhoods from R1 to R4 would basically 10x the density, let alone developing them to their highest and best use (i.e., multifamily, which would easily 10x it again, if not more).

Every urban single-family house on a modest lot physically displaces 10 families further out into the suburbs. Every urban mansion on a large lot physically displaces 100 families further out into the suburbs.

And you know what adds insult to injury? The owners of these properties are not paying 10x or 100x what people who own units of dense housing are paying -- not even close. Because the zoning prohibits developing to the highest and best use, it devalues urban lots compared to what they should be worth, and thus gives those homeowners a massive subsidy on their taxes! [Insert rant about how Georgism is better here] Urban homeowners are generally likely to be wealthier than renters, and the larger the lot they own, the more wealthy they're likely to be. So now our subsidy that's already unjust is regressive on top of everything else, subsidizing more the wealthier the homeowner is!

Frankly, people need to understand this and get fucking mad about it, because it's a goddamn unjust outrage just on grounds of equity, even before you get to the urbanism/walkability part!