They get copied by the copycats
Make is a whole lot easier to fix mistakes.
How is is even done?
The good thing is that we can fix those mistakes ourselves. Not necessarily easy work if other users (possibly your own distant cousins) have inputted incorrect data and relationships within your family branches.
I, like so many other Family Tree users, can spend many days trying to unravel a messed-up branch, so there will be a good deal of understanding of the issue you appear to be facing. Unfortunately, there is no magic wand, or dedicated FamilySearch team that can help you out, but posting specific problems here can often lead to tailored advice on how to fix things.
As for the wider problem (of what you describe as "myriads of mistakes"), when you don't have the time yourself you just have to walk away and hope, in time, other users will come along and put things right. Meanwhile, you have to accept this vast problem is not going to go away any time soon.
It is always advisable to have your own personal tree on another site such as Ancestry thus ensuring that the information cannot be altered, There are several free sites which enable you to do this.