Well, for a lot of reasons... if photography is what you're good at and what you enjoy, the make-money-on-ads model makes no sense unless you're also very good at generating traffic.
It also at least partially removes your income from the quality and quantity of your output. Of course the latter will be one important factor in building and maintaining traffic, but the reality of your business then becomes driving traffic, with photography as an ancillary requirement.
Plus you're now dependent not only on your ability to take good pictures and drive large amounts of traffic, you're also at the mercy of changing revenue. Great, you're making a living wage... but now suddenly your ad service isn't paying as much as it used to, so the same volume of traffic brings you less money. Oops.
There are tradeoffs, but I would not say this is a viable alternative at all unless you're also doing other things and driving traffic is as much your primary skill as photography.