Comment by darth_avocado

It’s a balance. You don’t want spam to be too much so that the product becomes useless, but you also want to let “promotions” in because they bring in money. If you haven’t noticed, they always tweak these settings. In last few years, you’ll notice more “promotions” in your primary inbox than there used to be. One of the reasons is increasing revenue.

It’s the same reason you see an ad on Facebook after every couple of posts. But you will neither see a constant stream of ads nor a completely ad free experience.