[tldrthelaw]

I think we have to take the question and flip it, as the law (mostly) lays out what you can't do, rather than what you can. If I own X shares in company Y, why can't I enter in to a contract with you whereby you take my X shares and return them to me on date Z?

If I can do that, why can't you sell those shares and buy them back (presuming I'm okay with that)?

It is just sort of the default position, and stems more from our property rights than anything inherent in securities law.