The reason that page views and unique views are harder to game is that they worth far less individually, while not being correspondingly easier to produce.
Say I'm a fraudster with a thousand IPs and a well-anonymized browser. If I'm paid per click, I can easily turn that into a thousand apparently legitimate clicks, which, depending on how I play my cards, could net me a few hundred dollars.
By contrast, if I'm paid per unique visitor, I can turn that into a thousand apparently legitimate unique visitors, which will very optimistically pull in maybe $5 (judging by Project Wonderful).
Now, neither of those schemes are optimal, but just about any improvement you suggest for the unique visitor model will reap ten times the rewards applied to the per-click model.