And how, exactly, do you propose to do that? Your body's own immune system can't do it, and it's had a 65 million year head start.
With cool nanomachines, of course. And the most part of that 65 million years wasn't spent on fighting cancer, but other stuff that would kill me sooner. Cancer is a relatively recent problem, I think. And even then, most of that time my immune system has probably been plotting against me, so that I wouldn't live too long and compete of resources with my offspring.