Comment by robotnikman

As a Warthunder player myself, that is basically the reason. Most players like the game to be as realistic as possible, and some players really like a specific nation. When they see a vehicle under-performing, they try to dig up data and documents confirming so, which is then passed to the developers.

Problem is, for modern vehicles a lot of this info is classified, but some players seem to be persistent enough to get that info and try to use it.