Why is 3 parking spaces per car inherently unsustainable?
This is nearing the square footage of an apartment to house the same number of people. So for any given unit of housing real estate we need an equivalent or greater amount of parking real estate.
When everyone wants to live close together (ie in LA near where they work, socialize, etc. not in middle of Nebraska) that puts a real pressure on space, and drives up the cost for real estate.
Even if you want to increase density by building higher buildings, the cost of those condo/apartment towers now must factor in a parking garage pedestal taking up 1/3 of its height, and so must offices, shopping centers, etc.
I don't think the ratio of parking spaces per car needs to be reduced. Rather it needs to be recognized, and thus reveal the cost of policies that lead to one car per person. If you can reduce the cars per person ratio, you reduce the number of parking spaces required at a 3x rate.
Cities are great because of all the different things you can get to. However cars spread the city out, and in this in turn means you can't do as many things. You can still do a lot, but eventually the distance (distances in cities are measured in time not meters!) is unreasonable and so you can't get someplace in your city. Dense cities enable more things. If you just want to go to walmart - so do everyone else so no problem. However if you have a niche interest you may discover not enough people live within range to support a place for your interest even though across the entire city there are enough people.
How are the cars limiting here? It's quite the opposite - you can have a special interest shop/venue in a cheap mall in some industrial area and get enough customers who can reach you by car. Without the car ubiquity you'd need to get into expensive real estate in the city center to get potential customers and charge them for your rent, which is fine if you are dealing in, say, collectible wines or artisanal Swiss timepieces and no business if you want to sell Gundam models or knitting sullies.
And that doesn't cover opportunity cost or the knock-on effects of market distortion, or the exponentially higher costs of e.g. parking garages.
Note that the above is about land area. Housing units are often stacked vertically, while car parking rarely stacked.
Where it is not stacked, it’s obviously not that terribly valuable.
Of course places where stacked parking is even a thought are also places where density is high enough that it isn't hard to run a great transit system and so there is less need for parking at all.