I used to think that, until I watched the TED talk by William Li [1]. The process of angiogenesis can be found as the catalyst in every cancer and medicine/food already work to reverse or halt angiogenesis. Doxycycline, for example, is a broad-spectrum anti-biotic that has been around for decades, is cheap and readily available and has been successful at slowing or regressing tumors. [2]
Yeah, these drugs appear to improve survival times somewhat, but at extortionate expense.
Avastin for instance has been something of a disappointment: yes, it appears to delay mortality but that's about it & the FDA has pulled its approval for use in breast cancer because there was no evidence that it actually helped. The side effects can also be difficult, as you might expect for a drug that targets a process like blood vessel growth.
If dietary changes could have profound effects on cancer progression then a controlled trial would be simple (and ethically straightforward). In reality, eating a decent diet is known to reduce the rates of cancer & improve survival rates of those who are diagnosed with cancer, but it isn't a cure & it probably never will be.
There's a lot to be said for getting to the cancer before it takes off. Silver bullets are easy to dismiss, but what we're really talking about is stopping a simple process with a simple process. It would be hard to stop a missile once it's off the ground, but from the silo it's a matter of turning off a few switches - or rather, not allowing them to be turned on.
I don't claim to know medicine, but I am familiar with simple measures equating to great results when acting on initial factors. An ounce of prevention equals a pound of cure in this case.
[1] http://www.ted.com/talks/william_li.html
[2] http://www.sunridgemedical.com/ResearchArticles/DoxycyclineC... (PDF)