Most free web sites pay for their upkeep with ads. It has been an unwritten agreement since forever (or at least as long as there have been ads on the web) that if you consume the content, you pay the creator by looking at the ads on their site.
Consuming the content without looking at the ads is like shoplifting because you don’t like the way a store’s checkout counter works and/or the fact that they want money from you at all.
Why? In Reddit you can’t reply if you get downvoted so there’s an incentive to flip upvoting and downvoting.
In Lemmy there’s no such mechanic so why flip them?