I get that marketing can't be done like that, so it makes sense but that is why some people go against the idea of showing only the big wins.
I don't know if showing a big win is a proper form of marketing or not but all casinos do that (if they have sosial media)and I think why they do that because one image is enough to tell everyone what's going on, differently if it has to also display losses requires more image views unless the casino has a summary feature so players can know the end result all of gambling activities they do, are in a profit position or vice versa.
It's absolutely the base marketing strategy for all casinos and you can check out that every casino shares some big wins on regular intervals on their Twitter handle as they have 30-40k followers over there and even if 1-2% joined the casino after such wins they are in profits.So they adopt various methods to showcase these sort of wins and try to attract more players to the casino.