A machine that's truly random will result in everyone losing money slowly and reliably after a large number of pulls. So not fun. A machine built to have the odds wander around over a period of time will be more unpredictable with a greater earnings spread
Also, for video poker and card games, the wins and losses are distributed as if it were a real deck of cards shuffled between hands. There are strict regulations there too, and payout percentages based on the rules of the game and the payout values for each type of poker hand.
For progressive Jackpots, the chances of you hitting the Jackpot on two successive pulls are the same as you hitting the jackpot on two pulls far apart. The payouts are different, but as you said, that is not hidden information.
Slot machines are able to use psychological tricks, I believe, such as showing "almost hits" with a higher probability than them actually occurring. "Oh, if that one symbol had just changed, I'd be a millionaire right now!"
State law allows them to change the odds after a machine has been idle for four minutes, and then they must not allow anyone to play the machine for four more minutes. During that time, the screen must indicate a change is being made to the game's configuration, said Travis Foley, laboratory manager for the technology division of the Nevada State Gaming Control Board, who is overseeing the Treasure Island test.