If Bob falsifies data and someone e-mails him, asking him to send them rope to hang him with, he can simply delete the e-mail.
Or claim he forgot the details of the analysis. Or claim it was handled by a grad student who left. Or claim the info was lost when a hard drive broke. Or claim the data was the intellectual property of College A and he can't access it now he's at College B. Or claim privacy or copyright rules cover the key data. Or that they don't have a license to the software that can open the data files any more. Or any of a dozen other things.
Typically bad research assumptions or implementations are rooted out during peer review, but it’s an imperfect process.
I do think there needs to be a dedicated non-profit and neutral organization solely responsible for reproducing scientific results for all fields, and assigning a reproducibility score to research finding. This could become an entire field by itself, and would have its own complications, but the reproducibility crises does exist and needs a solution.
Otherwise, years later, if the author dies, the paper would be basically worthless for reproduction.
(We're still talking about reproducibility crisis, right?)
This is how it works, people email each other all the time. Why shouldn’t you? You can’t imagine every bit of information that someone would want, and papers have page length limits so you make choices about what to cut.