Such a simple question; so many incorrect/incomplete answers.
Allowing such sales would break the airlines' "breakage model." This model depends on a very large percentage of such benefits (miles, vouchers, upgrade certificates) expiring unused, so the airlines only record as expense the cost of the benefits that are
expected to be redeemed. They count on this "breakage" and have no interest in allowing you to profit at their expense by letting you sell benefits that would otherwise go unused.
[Edited to add:] The previous post touches on this towards the end as a possible rationale, but in fact this is the sole reason.