There are more than 1 billion cars on earth. Cubesats are smaller than a car and can be distributed in an additional dimension (altitude). I think there is little risk of oversaturation currently.
Also, cars do not go 8 km/s (30 Mm/h, 17k mph). That really compensates the actual satellite size. And a single debris can ruin your day at these speeds.
The only reason I'd say cars are in a "better" situation is simply because we can clean up. We don't do it yet with satellites.
Cars are also not phased if they run over a loose bolt. The same can't be said for satellites.
> We don't do it yet with satellites.
Basic orbital mechanics indicates that we won't ever be able to clean up Kessler-syndrome space debris in a remotely-economical manner. There'd be too much of it, and it would travel at vastly disjoint orbits.
Orbit changes are extremely expensive.
Maybe a bolt wont break your car, but a nail will flatten your tire.
And most of the time we don't clean much the highways: a lot of small debris end up here and there...
The problem is indeed that cleanup is expansive and not interesting when it's not such a big hazard, but after some time the sum of those small hazards become a big problem, and the sum of cleanups almost impossible.