I think they do get won, but the jackpots aren't only for 888, many sites have these games so the jackpots, when won, aren't likely won on 888.
Obviously the jackpots are won very rarely and add in the number of sites with the games it's even tougher.
Even the guaranteed daily jackpot is spread over several sites I believe.
Like you I pretty much try to play the casino just for the casino mix-up ticket or when I'm near completing a level, or if I have a bonus.
Currently I have an $8 casino bonus from a club scratch card up to $199 with only $141 left needed to wager to clear it.