This means that cliques along the lines of "I'll scratch your back if you'll scratch mine" form naturally. It's almost impossible to prevent as a lot of the trading happens directly between traders and never crosses exchange boundaries and even then, it's often tricky to prove collusion purely based on the trades. Official trading communication channels are monitored, but this is irrelevant if the deal can be pre-arranged privately off channel.
There are too many conflicts of interest in the entire industry really. It's not in the best interest of the banks to monitor their internals too closely. Sure, there are plenty of theatrics and checkbox-ticking exercises, but really, as long as it is cheaper to pay the eventual fines than to employ armies of auditors and actually give them the motivation and power to enforce the rules, it's not happening.
The internal controls themselves can be fairly annoying too. I remember a 6 month effort to get our legal department to clear a US fund launch because their incentive was to just reject every proposal. They don't directly get to see the benefits of a successful product, but would certainly feel the pain of one that gets into trouble for whatever reason.