Because until the Wachowski sisters brought in the "Neo is the One" plotline with the attendant messianic tropes, the relationship between Morpheus and Thomas Anderson/Neo was one of a union organizer showing a worker blinded by false consciousness exactly how he allowed himself to be conned by his bosses.
That's easy. That's why I'm fat.
The point is "gives rats cancer" bascially has no meaning. As another example chocolate is poison to dogs. Does that mean humans shouldn't eat chocolate foods or drink chocolate drinks?
Humans also eat plently of things that are toxic. Most spicy peppers are toxic if eaten in the quantities needed to induce by other substances in rats. So is vinegar, wine, cheese, pickles, ...
Ever heard of that super toxic dihydrogen oxide. About 360,000 people die every year from it, some with as little 120grams (1/2 a cup)