I currently live in an apartment building of 25 people and am closely surrounded by similar apartment buildings. This means that there are literally a couple hundreds of people than can annoy me at any time - either by listening to loud music, by doing some house renovations, by leaving a constantly barking dog at the apartment, by parking a loud car/motorcycle near my window, by having loud conversations near my window etc. In the suburbs, you get similar factors (+ most likely more lawnmowers and leaf blowers than in my district), but you only need to worry about perhaps 10-20 people creating a nuisance, not hundreds. Not to mention that, in an apartment block, I am often able to hear (over the walls) conversations happening in surrounding apartments, while this is just a nonissue in the burbs.
If burbs people complain more, it might be because they expect perfection (they bear the daily long commute so they want to get real peace and quiet in return), while apartment dwellers just live in resignation.