Because there still exists informational asymmetry where the platform/exchange owners (and all employees privy to that information) still have material advantag

Because there still exists informational asymmetry where the platform/exchange owners (and all employees privy to that information) still have material advantage over their customers but can now manipulate them easier than they can in traditional systems. With traditional systems, you have to purchase shares with your real name and thus regulators are able to detect patterns easier. In DeFi, these guys just spun up new anonymous wallets each time and if they were slightly better at it, they likely never would’ve been caught. Getting to parity of current insider trading detection in DeFi would require massive data analysis constantly keeping track of which wallets make suspiciously timed trades, leading to the same financial surveillance and de-anonymity DeFi wants to solve. One good tweet is not enough to validate the whole system.