In practice, this makes these networks very resilient to fraud.

Could like, 51% of the owners just coordinate to kind of, do a fraud?

Sybil attacks sound like the kind of thing you’re talking about. I don’t have the expertise to go into it, but one person (or a group) creates lots of nodes and uses that influence to do bad things to the network, potentially including fraud. Or as you suggest, legitimate users can just coordinate to do whatever they wanted (see ethereum vs ethereum classic if you want a chuckle).

I want to make a note that the networks are only resilient to a specific type of fraud - people trying to enter data in a way that doesn’t meet the criteria of the system. That’s all well and good for wallet to wallet transactions, but when you have transactions going off chain (like buying something, trading for other kinds of coins, doing anything with crypto exchanges), there are still plenty of other kinds of fraud that are possible and happen all the time, because while the chain is fairly trustworthy, nothing else about the system is. Most kinds of fraud involve doing things that technically you have permission to do, because you lied to people to access their password or promised them bigger returns in the future or missold a product or service etc and all of that is still possible under crypto. In some cases crypto is more vulnerable to these things because of having no central authority or regulator or laws or whatever.