The thing is, it's not just about the payouts. Yes, there are people that use the casino to grind, but keep in mind, those players enjoy getting units and items, not the casino itself, and would switch to the next most profitable activity if casino got nerfed. Doing that would also present a new problem, since the casino isn't all that amazing payout-wise, if we don't count the jackpots.
However, as I said, it's also about the activities themselves. The casino is well-made, laid-back, exciting and pretty unique as far as videogame experiences go. If other activities don't get the traction it does, it's also because they aren't all that atractive to players (let's face it, if they were, more people would play them without a compensation).
Finally, I think many of the examples you pointed out are in fact already pretty balanced. Yeah, there are some activities I think could use tweaking, but most of them are fine. However, deliberately buffing unpopular activities wouldn't be fair to people that enjoy the rest of the game's content and it could make big chunks of the playerbase do activities that aren't all that great to begin with.
(Replaced my previous post with this one)