As an example: Some dumb ass Verizon customer gave them my email address as their own. I got a ton of communications from Verizon that I could not turn off (bill past due, thank you for you promise to pay, and all sorts of really annoying messages). I called Verizon, I was told that neither them nor I could do anything about it because I did not own the Verizon account, and I did not have the account PIN. Their official solution for this problem: Mark all the Verizon emails as spam and don't call them again (I kid you not).
The result: I marked about 100 Verizon emails as spam and Gmail learned that really well; now I do not see Verizon emails in my inbox. Why TF can't gmail do this with all the other emails I mark as spam and not spam?
I just got in the habit of firing up the hideous gmail app on my phone to go through my spam folder every day and vet out the non-spam out of there. I usually go though about 250 emails per day. I end up looking at all the spam, but it's far faster to fish out the good messages from spam than if I turned spam detection off altogether.
No matter how complex the problem to solve is, Google is failing at a level where even I could program it better, which is not a high bar at all.