Because they almost certainly can not. They almost certainly have no idea what the consequences of their actions are as the drug company almost certainly did everything in their power to obscure, obfuscate, and downplay the risks as literally every company is legally allowed to do.

The principle at play here is “informed consent”. Unfortunately, modern society has little interest in the “informed” part which is why it is probably not a good idea to let people imbibe untested drugs even if they are giving “consent” unless they can demonstrate they are “informed”.

Because I don't want to live in a country where desperate people can sell their lives to soulless corporations for literal cash. That's some real dystopian bullshit. At the extreme end of this spectrum is the most dangerous game, where homeless people agree to be hunted for sport. And the topic we're actually discussing is pretty damn close to that end of the spectrum.

Additionaly, medicine companies aren't going to waste money trying stuff they know is likely to fail (like the example "radium asbestos")

Mentally able =/= intellectually sophisticated to parse the legalese in the offer