Hextech crafting can't replace needing to spend RP on skins you actually want. Riot is a company that wants to earn a profit after all.
Why would I buy a skin, if I can simply trade a few unwanted shards for the shard of the skin I want?
Similarly, hextech chests and keys are another source of income for Riot (for people to spend RP). They work largely because they're extremely discounted BUT you don't necessarily get what you want. So people need to buy a lot of them to be satisfied, and it results in overspending (so much so, that Riot put a daily limit on hextech purchases).
If all of a sudden you could trade the shards, people would start doing that instead. There'd be no reason to reroll, just trade. There'd be no reason to buy skins, just buy chests+keys and trade the results. Etc... etc...
It would significantly lower their income, which isn't what they want to do. Think of Hextech crafting as a fun way to get free stuff, also giving you multiple incentives to try more champions (first to earn more chests, and second because you may have new skins on them!). It isn't meant as a means towards obtaining that hot skin you want. It's a simple reward system to supplement your normal skin purchases (or to give free players access to skins).