You asked "Why can't the SEC send people to jail?", to which the answer is simply that they have not been granted that power -- whilst there is no law specifica

You asked "Why can't the SEC send people to jail?", to which the answer is simply that they have not been granted that power -- whilst there is no law specifically restricting it, the SEC cannot act ultra vires its granted power, and doing so without authorisation would come up against false imprisonment, human rights etc. IF you are asking "Why hasn't the SEC been granted the power to send people to jail?", then that's a deeper philosophical question. There would be a concern that the diffusion of such powers could be abused or wielded by non-judicial-experts who aren't necessarily acting in the interests of justice and broader public policy, and more generally the principle of separation of powers discourages such to prevent tyranny/the abuse of state power.