I'm not sure that is the case. If everybody lived in apartments then there would be more people in your area which would lead to you seeing more people along with worse traffic.
People who live in areas with like 5 people per square mile don't complain about there being too many people around them because nobody is around them.
To an extent, though the sensation of crowding can also diverge from the actual population density depending on how the city planners organise things.
Berlin feels much more open and chill than places in the US; even the Mitte district whose population density is 9,600/km^2 and which is only a bit less than the 11,313.8/km^2 of NYC[0] and quite a bit more than the 7,193.3/km^2 of SF.
[0] whole thing not just Manhattan which is 28,154/km^2
For example, Fresno is basically the same as El Paso, Texas when it comes to climate, but has outrageously higher taxes and cost of living.