Taxing proceeds of a crime doesn't necessarily compensate for the damage of the crime. The same applies to some of the greyer areas of finance. An individual can gain millions and pay fair tax on them whilst wiping out billions in value for others. Especially when it comes to insider trading where it's much easier to induce an artificial fall in a share price than an increase.
Sure, there is a certain strand of right-wing economist that has got the wrong end of Coase's argument about transaction costs would argue that virtually all forms of financial shenanigans could, in theory be solved by investor consortiums that stand to lose billions paying the bad actors more than the millions they could earn through malicious activity to desist. But I don't think any sane person believes that shareholders agreeing to overcompensate executives even more to avoid them seeking to profit from insider information at their expense actually is a more efficient solution than aligning those incentives through the legal system.
Sure, there is a certain strand of right-wing economist that has got the wrong end of Coase's argument about transaction costs would argue that virtually all forms of financial shenanigans could, in theory be solved by investor consortiums that stand to lose billions paying the bad actors more than the millions they could earn through malicious activity to desist. But I don't think any sane person believes that shareholders agreeing to overcompensate executives even more to avoid them seeking to profit from insider information at their expense actually is a more efficient solution than aligning those incentives through the legal system.