There are many reasons why it doesn't work.
Imagine you're about to get perma banned, of course you would want to save your skins and all before it happens lol.
You can get 4 keys and 4 chests for free each month. Transfering skins and such would just end up in many people grinding new accounts for the free stuff and transfer them all to the main account.
I think you see now why riot will never implement that.
Besides these two reasons, making it able to transfer skins to other accounts, would make it possible to gamble these things like cs:go skins and thus more illegal websites would open and more kids would get scammed.
I fully support riot with the decision not to transfer skins.