I agree, it's just that you're not the average user. I am also surprised to see the spam folder surprisingly empty all the time, almost as if my email addresses were unknown to the world (which I am pretty sure they are not).

The average user probably sees this full spam folder as something he/she is doing bad, or the service itself is bad.

It's not just google or microsoft. I think most email providers do this, I might be wrong though.