cars wear out from two things
1) age
2) use
A car, left sitting in the driveway for 30 years, unused and unmaintained, is unlikely to work very well or for very long. Rubber components like hoses, wire insulation, weather stripping etc become brittle and break.
As some other posters mention, cabs with 650,000 miles are not unheard of. I had a Toyota Landcruiser with 450,000km on the second engine, over 900,000km on the body.
A car that got 20x use would not wear out 20x as fast because a large part of a car wearing out is just age, not miles.
There's a Volkswagen Gol here in Uruguay with a million kilometers, and it's not an unheard-of amount
Cars are the most expensive in the world here, so we tend to keep then way long past their expiration date - as an example, I own a 1994 Maruti with 200.000 km, they aren't designed to last that long ! Japanese cars are the most coveted because they do last a million kilometers if cared for properly.
Sadly, there's a ban on used car imports (The vice-president's campaign was funded by the new cars importer association).
A million kilometers is considerably less than a million miles. A million kilometers is about 621 thousand miles. That's a lot, but it's much less than a million miles.
With my comment, I just wanted to add one more anecdotal point :)
I'm aware of the difference between miles and km (though I instinctively tend to minimize it and believe the difference is less than it really is)
I know of that Volkswagen because a million km is headline-grabbing here (on the "anecdotes" section), there are probably cars with a million miles but 1,609,344 km is not a headline-significant number, much like 621,371 miles isn't for the US.
I did actually find this quite helpful, as a Canadian I usually gloss over the km/mile conversion with a nice 1 km ~= 1 mile, as with most things you talk about (speed < 60, distances less than 100) the difference is fairly miniscule (and handwavey!). I didn't realize 1million miles is only 621,000 km! Thanks from an ignorant Canadian :)
As another Canadian, it's annoying to have you link your ignorance to your nationality. I'm confident that the vast majority of Canadians know that a mile is, very roughly, around 50% longer than a kilometer.
And if you think the difference is miniscule in normal usage, you just try driving 59 MILES per hour on a city street with a 50km/h limit, and see how the cops feel about that. (I say 59 because my lived experience is that everyone drives 10 over, and the cops don't ticket at 9 over; your city may vary)
Or try estimating when you're going to arrive at a meeting that's 100 MILES away on the highway, when you think "oh, 100 km, I can go 110 on the highway, plus the time to get out the door, call it an hour".
Even at walking speeds, 2 miles of walking is going to feel different than 2 km of walking.
Whether urban legend or not, supposedly there's a mercedes "million miles" club. They did famously buy back a mercedes that had over 2 million miles on the clock that's in their museum now
Rust qualifies somewhat under #2, but not completely. Rust isn't 100% preventable in some areas. I live in Michigan and because we use salt on our roads, if you're not washing the vehicle (along with the undercarriage in particular), you will inevitably get rust. On my last vehicle the engine mounts (which were part of the frame -- unibody) rusted. There was pretty much no way to fix the car soundly. If I had washed the underbody frequently enough I could have saved it, but there is a cost associated with that as well.
I recently saw something pointing out another wrinkle about taxis -- per mile, their engines go between hot and cold a whole lot less, so there's less stress from thermal expansion. Once they're on, they pretty much stay on for at least the driver's normal workday.
I don't know how much difference this actually makes, but common knowledge amongst the people I know seems to be that the most stressful time for a normal engine is starting up.
It's not just common knowledge, it's the truth. And it makes a huge difference.
When an engine starts up, all the oil is sitting in the bottom. In a good condition engine, the oil pump starts giving meaningful pressure the moment the starter turns, and starts pumping fresh oil throughout the engine.
However, when cold, an engine has the wrong tolerances to account for when the materials heat up and the materials expand. So the oil pressure isn't quite right.
As the engine ages this problem gets worse, so each startup cycle gets progressively worse. This is why a car with a worn engine will show the 'oil pressure' light for an increasingly long time after it's started.
The length of service life for an engine will come down to
a) operating hours (not just distance)
b) operator abuse (revving while cold, excessive RPMs throughout use)
c) service attention (oil changes, filter changes, coolant changes)
d) duty cycles (how many times it heats up and down).
The worst thing you can do for a car is a lot of short trips with a big enough spacing to let the engine cool, and aggressive driving while the engine is still cold.
An F1 engine is seized when cold, it requires several hours of warm water and oil to be pumped around to bring up the metals to the operating temperature.
Modern engines can go a very long way if cared for properly.
As some other posters mention, cabs with 650,000 miles are not unheard of. I had a Toyota Landcruiser with 450,000km on the second engine, over 900,000km on the body.
A car that got 20x use would not wear out 20x as fast because a large part of a car wearing out is just age, not miles.