I don't understand this phenomenon either.

I self upvote but my upvotes are worth practically nothing, and sometimes to improve visibility.

But for people who have earned/invested thousands of steem, rewards from self upvoting shouldn't mean much (because they already have so much) , instead to maximize the value of their investment , they could create /curate content and advertise. Self upboting is not even in their own self interest for the long term.