Hrm when I played PS2 I always felt the game was amazingly cheater free. Relatively speaking of course. I'm sure SOME existed but it was nothing at all like counterstrike games where every few minutes there might be a dude zipping around at 10x normal move speed ruining everyone's fun. As for why they do it, who fucking knows. Hell, playing GTA3 ages ago I had fun for about 20 minutes with invulnerability cheat on in single player laughing at FBI cars endlessly crashing into mine, till I turned it off and played the 'real game'. No clue why someone would want to do that on a live server against actual players, angsty douchebaggery maybe?
I played on Waterson and would hear this or that about a Connery or Mattherson clan and go roll an alt to see what the fuss was. The accusations were always bullshit, tough dudes but beatable, which is how top guys should be. There is a high skill ceiling for stuff in PS2. Every good shot gets called out from time to time but when a lib is flying backward or upside down and you get DOMED by a single belly gun most are too prideful to laugh it off, and realize that gunner or pilot simply knows how to do shit the other guy doesn't. Then come the tears...especially when the pilot of the lib ... was also the gunner!