Then they'd be sued into oblivion, because they would be admitting their product is dangerous. And since the 50s (and before, but it ramped up in the 50s), the industry had spent millions on trying to convince everyone from the public to the medical community that smoking wasn't dangerous, even though their own internal studies time and time again confirmed the studies which showed the dangers of smoking.
This is the same reason why they couldn't market a "safer cigarette", since that would imply the product they were selling before was dangerous. The tobacco industry was afraid of litigation for decades before it became a reality, and this prevented them from pursuing options like e-cigarettes, which actually are safer "cigarettes". Incidentally, e-cigs were regulated under a different set of rules than cigarettes, cigars, etc. which prevented other companies from marketing them, since nicotine was considered a dangerous drug, even though it isn't the nicotine which is causing the lung cancers, heart diseases, etc. (outside of the addiction it causes).
Couldn't they have just said, "You knew what you were getting into when you started"?
That's one of the biggest arguments they use in court cases, and they've actually lost several times using that argument since a common counter-argument is that people actually didn't know, given the information available to them. Even after the Surgeon General's report, many Americans weren't sure that tobacco was actually dangerous, in large part due to the campaigns Big Tobacco was behind which repeatedly stated that "it isn't proven" that smoking causes cancer, heart disease, etc. It's a sort of false neutrality which psychologically appeals to a lot of people, hence why controversial groups from creationists to the oil industry have used the same tactic.