Although they are all bigger or smaller scams, they all survive because their customers are either mentally incapable of spotting the scam, gullible, naive - or suffer from gambling addiction. They all manipulate, that's their very nature and sole source of revenue.
Do you have any proof for this? My friend plays on Prime-dice every week or so. So far, he hasn't incurred any loses at all. I have studied the results from hundreds of games played by him. I found nothing suspicious.
If it's provably fair then how can they manipulate?
All smoke & mirrors, nothing more. And if someone takes an edge on a game and lets you do the math - he still has an edge and is tilting the table (falsely interpreted as honesty by gambling addicts).
*hint* showing long lists of games does not mean this list is complete, there are many ways to present you with something you can calculate and be apparently correct, for as long as the house can add unlimited virtual/invisible sessions unknown to you - they can create any matching checksums for any lists they like...
Plus, noone can "break the bank", the most legit services hold back their manipulation algos for the most profitable targets and to protect the bank (those will gladly break any luck streak if a winning streak endangers the operator).
Common sense or knowledge is often absent in gamblers minds, together with the number of addicted it makes for very profitable targets. It suits the BTC world well I must say so I don't have a hughe issue with their existence.