There is no manipulation or related thing about that, period.
These game software providers are also taking care of their business. Issues about manipulation will just create a bad image for these providers. If there will be an issue of cheating or manipulation against them, how will they be able to attract more online casinos to hire their service? Their games are fair and square and they are distributing these games with the same system throughout different online casinos.
This argument of "too big to fail" is proven to be wrong throughout history. The trusted 3rd party still can be malicious if the safeguard isn't there or not correctly implemented (or many other reasons), the license and stuff are used to prevent such malicious action. Therefore, such suspicion is common, and that's why people invented provably fair mechanisms to erase such suspicion.
The bottom line is that the game provider can or cannot be malicious, even if the manipulation risk is low, it's not zero. People playing those games are basically ignoring the risk and choosing to trust the provider.
But of course, people who blame the system, because they are losing, aren't correct either since it's gambling... They must have a proof for such an accusation.
The question I have in mind is, aren't the developers the ones that should be responsible for any manipulation in the games or it's still possible for the casino team and members to manipulate the game engines?
Some people still think that games on online casinos are from the casino teams themselves but those games are from software providers, is this valid or not.
There are two kinds of games based on who creates them, (1) in-house, and (2) 3rd party providers
For in-house games, the house/casino builds the game, so they should have the burden to be liable if any error, but for 3rd party games, the casino only hosts it as is with some parameter to tweak, like RTP (for some games) for example. I don't think it's viable for the casino to hack the provider's server and stuff, it's simply not their expertise. The most common MO for casino to cheat their customers is by denying withdrawal, as simple as that.