Although I have also experienced this several times in the upper Midwest. I suspect there might be some bias in my physical appearance that makes me get offered more? Several drivers had a sort of setup where they had goods in a box/console and opened it to make the offer, so it seems likely that if they didn't feel a fare to be the partying type they would simply not make the offer? One I saw had an entire cooler setup.
Another possible explanation could be that in very competitive areas like LA or the Bay the rank/yank system cuts out bad drivers more quickly than middle-america where there is maybe not as large of a pool of people applying to be drivers?
I have never had a bad or sketchy ride in Bay/NYC/Chicago/Dallas. (I have had no experience with Uber in LA)
I have known several drivers who sold marijuana, small individual shot bottles, smoking papers, flavored cigar papers, and other 'party related favors' out of their cars to make their Uber driving more profitable. Without this source of income made from people going to/from parties and bars on weekends, alongside their actual ride fares, their profit margin is not even worth the time.
These 'tipping' hustles are frankly extremely mild in comparison to my experiences. I'm of the opinion that these hustles are publicly frowned upon, but in practice are not effectively policed on purpose. If Uber policed these sorts of practices in an effective way(like if they had cameras/monitors in cars) they would lose a substantial portion of their drivers instantly.