Great intention, but too simplistic. The lawmaker has a third party set up a trust for the benefit of his children and the trust owns a company in yet another jurisdiction that just happens to not only take a position in a certain stock, but probably provides "consulting services" for a healthy fee. And that is the simple version. By the time the lawmaker's interest is fully obfuscated, the legal structure is fully compliant with every letter of the law, just not the intention. Better to have fewer laws that are actually enforceable.
That's like saying "why bother banning insider trading? People can just obfuscate their illegal trades".
Yeah, sometimes people can break laws and not get caught. But that's no argument against laws. Some congressmen might be able to hide their investments, but sometimes they'd get caught and plenty of them would not take the risk.
There are already insider trading laws which almost everyone has to comply with, congress people being an exception. They specifically allow insider trading for themselves while prohibiting it for everyone else.
If you work in a financial company which does trading often times you are very restricted in what you could do. You also have to report all of your accounts plus close relatives accounts.
And if you have an insider info and give it to someone else, no matter how remote from you it's still against the law to trade based on that and it's not "fully compliant with every letter of the law"