Some lowly paid person at Google can look at your site later today, decide that (in his/her opinion) it isn't good enough quality or you've optimised your CTR too well and your adsense income comes to a full stop with very little chance of you ever getting it back again. It happened to me a couple of years ago and it's one of the best things that could have happened.
I did some deals with merchants that used to benefit from the ads on my sites and now I get a much better EPC, much more control over how I do my marketing, far more security since I'm dealing with several companies instead of one so if any of them mess me about I can switch the business elsewhere within minutes. Adsense? No thanks. I'd rather deal with Beelzebub himself than Google.