In walking cities you interact with the city itself -- you can stop and look at things closely, walk into a small store you've never noticed before, run into a friend and chat, or pause and just people watch.
In car cities everything's far off and viewed through glass, your only interaction being which direction you point your car and how fast it moves. And becoming distracted by something interesting causes expensive and dangerous accidents. Plus, driving is inherently frustrating and exhausting, in my experience, especially when compared to walking which is at least a nice bit of exercise. You just want to be where you're going and WHY IS THERE ALL OF THIS TRAFFIC?
Or there is a change of jurisdiction, and one didn't require sidewalks and one did, often city vs. unincorporated county land.
In one way this makes sense since almost all customers arrive by car but it certainly makes the street visually unappealing.
EDIT: I wrote "arrive by foot", obviously it is arrive by car in suburban locations.
Lip service - just fooling the planning departments which sometimes seem to fall for the dumbest ideas.
https://en.wikipedia.org/wiki/Commissioners%27_Plan_of_1811
Lower Manhattan has plenty of nooks and crannies. My father had a secret street that he would park on when he needed to visit Battery Park City. It was a little-known street maybe one or two blocks long that was perfectly legal to park on but that hardly anyone knew about because it didn't go anywhere. I went with him a few times, and we were the only car to park there.
Plenty of nooks and crannies can be found, even in grids. There are always irregularities and sacred cows to be worked around.
Also, Chicago has a lot of nice nooks and crannies. Including an ordinance that requires that a cattle path be maintained through some of its downtown skyscrapers, and an old barn at the end of an alley.
https://www.chicagoarchitecture.org/2009/04/08/the-loops-mos...
A good example of this is parking minimums, which make Manhattan-style density literally impossible in places like the SFBay.
We're obviously living in some of the safest times there's ever been, but that probably plays on some minds (alongside the "it's more efficient to build straight lines" part).
Part of this might be necessitated by the fact that we build most structures out of flammable materials, but with better fire-control technology (sprinklers) that could be mitigated.
It's amazing how often alleys/laneways are some of the most treasured parts of a North American city.
My city has caught on, and there are a bunch of car free streets and alleys being made in the CBD. They have boutiques and restaurants with outdoor seating, and it changes the dynamic of the area entirely. People are relaxed, not rushing, chatting with eachother without yelling . You get a lot more friendly encounters with strangers. It feels like you've reached some kind of special community area, just for you and the others there.
Oh the irony.
Who says they can't? The question being asked was "why are there no nooks and crannies". Those by definition only appear in dense places. You can't have a nook in a large empty field.
There was never a time where the average city dweller had a horse, let alone two or three. In 1915, there were est. 20M horses in the US, for ~100M people. There are way more motor vehicles per person nowadays.
It's been popular with plenty of folks for some time, like libertarians, fundamentalist Christians, environmentalists, etc. - I notice it's becoming more common among people concerned about climate breakdown, who are probably concerned that the support structures which make cities possible will fail.
Calling it unsustainable probably refers to half-assed homesteading, where you basically do some gardening on a big patch of land but still drive everywhere and use lots of energy. Homesteading (grow your own food, coppice trees for heating fuel, solar panels for electricity, ride a mountain bike to get around, etc) can have a very low carbon footprint.
Just doing the quick ugly napkin math for rough validation, Wikipedia says the world has roughly 14,000,000 km² of "arable land". If you convert that to acres and divide by the current global population, that's a little under half an acre per human. If the average family grouping were of size 4, that's only 2 acres of supposedly-arable land somewhere in the world per family. That's skipping over a whole lot of details and side-points, but still, it doesn't seem like this is a sustainable path for the whole planet to go down.