Some of them could be lying too since it's so easy to do it on the internet where nobody would be able to track you down. But it's also the nature of the sport, it's highly competitive and bound to bring out the worst in people. In fact, I don't see anyone who's truly and deeply intelligent taking part in any of these discussions/arguments let alone trolling on any forum. That would be a sharp contrast to what we call intelligence.
Another thing that factors in is the so called belief system. If, say, a Federer fan or even a Nadal fan (I believe they'll get there too) believes their player is the best player of all time or the so called GOAT, then no amount of argument is going to change their mind or see things otherwise - no matter what you present in front of them. A belief system is that strong and tend to be that blind, unfortunately.