That is true, I've often felt the same way when reporting someone. They want visual proof, but how can I give them visual proof of a speed/fly hacker when by the time I make the proof he's already gone? And then also, sometimes proof can only be done with a video - so I have to record a video, then upload it every time I see a hacker? This will kill like half of my game time in that case.
They say "we'll investigate further" - okay, but I wanna know if my report has been in vain or if something has actually been done. I dunno, it would be really nice and satisfactory if we players could get feedback when an actual action is taken against someone we've reported, like: "your report is invalid as this is not against the server rules; we apologize but we couldn't get enough proof; we are happy to inform you that following your report player X was found guilty of breaking the server rules and has been punished". It's not much - it's very simple, yet very satisfactory - that's the art of feedback. And it will give players a lot more trust and reliability in the GM's capabilities.