If insider trading is allowed then, yes, those who have access to inside information can trade on it more efficiently. But it also highly incentivizes market participants to hide information. In a world where insider trading is allowed, anyone who has access to inside information can now personally monetize it as long as that information does not get out to the wider market.

You're basically paying people to manufacture information asymmetry. As an emergent effect, that does not lead to the widely available information needed for a market to perform efficiently.