You're not wrong, but Waze directs traffic down roads that aren't considered main thoroughfares. That's part of the issue. There are paths to work that I would have never imagined had I not used Waze, and plenty of those routes take me through quiet neighborhoods.
A good example is the shunting yard I encounter. Most of the time, trains are short enough that if they stop, they don't block the crossing. There is a short way through a neighborhood that adds a couple of minutes to your commute (in either direction) that dumps you out on a highway just on the other side. There are also two longer routes (one for north, one for south) that goes around neighborhoods but adds 10-20 minutes to your drive time. I wouldn't call the short route a main nor major road, but I'm sure the residents who bought those houses more than a few years ago didn't know that the uptick in rail usage would cause much more activity on their street. You can even cut deeper into the neighborhood to save minutes by circumventing the line of cars.
On a related note, I stopped using Waze after watching someone roll through about 5 stop signs. The driver didn't hit anything, but that moment made me realize that I was part of a neighborhood nuisance that nobody asked for.