I'm guessing where inventory is short (San fran, Seattle, NYC) is not where three car garages are being built...
You would be surprised. A lot is related to really stupid/outdated/car-centric zoning that forces/enables a lot of these things.
The exact case you mentioned is probably right, but for example when I was in San Jose, I was trying to do some renovations which would have essentially created an extra guestroom/office/in-law suite at the expense of an old garage. The dwelling itself was fine, but I ran into trouble with minimal covered parking requirements (and some more stuff about minimal driveway lengths). This was in an area that was in biking distance of downtown and the central transit hub.
I had given up at that point, asked a local staff person about it later, they kind of shrugged it off and admitted most of the codes were written in a hurry in the 70s, when energy was cheap and everybody loved suburbs and car oriented design. Nobody really wants to go through updating that stuff, it's contensious, unsexy, and doesn't get you re-elected.
When you dig into the stuff about not building smaller & more affordable apartments, you run into zoning a lot as well.
Tangential to the point, but fucking zoning stuff.
Oh well, but there's another side to that. Here in town, there's this new push for more workforce housing, because it's expensive as shit here. So under a new "AUD" plan (Average Unit Density), certain areas of town are being given the opportunity to increase density to provide more housing.
There are problems with this, of course. In my 'hood, and many other hoods, there is a requirement for each unit to have 2 off street parking spaces. One covered, one uncovered. Now, some houses have approved exceptions (like mine, because the lot was split in the 1950s and the garage went to the newly built back house). So my house is required to have 2 off street spaces that are hidden by a hedge.
Anyway, now there are lots that are zoned "R-1" (residential, 1 unit) that are being approved for more units. So you'll have a nice old couple in their 850 sf 2 BR house...and suddenly, right up to the lot line, there are three apartments that are 2BR, 2BA...but with only *one* off street space per unit.
I can tell you, each unit is going to come with AT LEAST two cars, if not 3. I know it SOUNDS nice to say "we don't need that much parking", but the honest truth here is that the town is just going to make it worse and worse.
The only place I've ever lived that was better than this was the DC area. I had an apartment. I got one spot. If you wanted a second one, you had to pay for it, and there was a waiting list. The city streets around the building had zoned parking, and you needed a permit.
This town is not like that. People have cars, and they drive them, and ignoring it is just really stupid.