Because death is not technically a disease, right now if a company came up with a literal immortality pill they wouldn't be able to get it patented or get FDA approval for it.
If you invent a pill that gives old people the mental and physical vigor of 25 year olds, you don't need to patent or trial it as an immortality pill. Just pick one of the many medical conditions associated with advancing age, like osteoporosis or male pattern baldness, and prove that it safely and effectively treats that condition. If the pill cures baldness and also gives patients improved muscle tone and working memory as a side effect, the FDA won't complain about those beneficial side effects.
Once the drug is approved for treating baldness it can be prescribed off-label for other conditions and/or the manufacturer can progressively run trials for treating additional conditions, similar to how GLP-1 drugs originally developed for diabetes treatment have now been tested and approved for treating obesity.
Why can't you patent a drug that maintains teleomere length? Or one that slows down metabolism? The FDA does not control the patent office, merely what drugs can be sold in the US. And neither of these inventions seem like they would fall under the limited patentability exclusions.
So invent it here, patent it everywhere, and sell it in the countries that allow it to be sold.
Because death is not technically a disease, right now if a company came up with a literal immortality pill they wouldn't be able to get it patented or get FDA approval for it.