Because they track you and don't care about security or privacy.
Also they lack support for updates and devices get fast EoL.
(In the past I own a OnePlus 2, my girlfriend the 3T and a friend even the 1, so I know what I'm talking about).
Okay I didn't know about that, because I have never thought about buying one, so I have not researched it. But I am just impressed over the hardware they use. But if it mean you will get tracked and I dont know what, then it's one step ahead and 2 back. But then, it's the Chinese. You need to keep them in a tight leach.