The same reason we put 2-3 of them in the same letterbox or we trashed some packs at the end of a street (yeah I did that back in the '90s as a teenager). To work less and earn the same.
I mean at that point you could just put the entire stack in the nearest recycle bin and call it a day. Much more efficient! Much less of an annoyance to people.
But that would be easily discovered. It had to be "good enough".
I have no idea why they bother but the sticker on my post box and those of some neighbors were removed the third time now in the last 2 years. I know that they didn't remove them themselves because I talked with them and gave them new stickers. The kids in the house are either too small or to old to remove them so I can only assume it's the people distributing flyers. It sure is not the postman, he actually wrote a note asking everyone to put up those stickers because he didn't want to carry stuff that ends up in the trash anyway.
If you are paid by ad distributed wouldn’t you want to try to maximize the number of ads delivered in to the same apartment building?
Not through vandalism and scammyness, but maybe that's why I don't work in advertising.
I've had door to door solicitors argue with me, after I pointed to my quite prominent "no soliciting" sign, that they weren't really "soliciting" but, rather, "marketing" and therefore the sign didn't apply to them.