What happens if Q proves himself without a doubt, like if he says Trump's first tweet tomorrow will contain the words "apple pie" and then it does? Well, we instantly go from being some conspiracy-minded weirdos to being The Place to get the inside info. Instead of some people hanging around because they think we might be onto something, we'd soon have the entire media industry looking over our shoulders, either hoping to catch the next hint or hoping to stop it. If you think a few e-celebs and the shills who accompany them are bad, multiply it by a hundred. And instead of bots and shills and bad mods sliding threads and occasionally deleting them, we'd probably just get shut down altogether, because too many people would be watching.
tl;dr To do what he's been doing, Q pretty much has to stay deniable. That's part of it.