I think that if there were a law in place that made it illegal for congressmen to trade, it would be easier to catch insider trading because they would be forced to trade by proxy like you mentioned.

It's really hard to prove someone traded off of insider information if they make a decision to buy/sell for themselves. If they don't tell anyone else why they made the trade, you don't really have any proof that it was made off of material non-public information.

If a congressman is forced to ask someone else to trade for them, they might be careless and you might be able to find evidence via text messages or phone records.

The last time a U.S. Representative got convicted for insider trading, it was because he called his son to tell him to trade a stock that he had insider information on: https://www.justice.gov/usao-sdny/pr/former-congressman-chri...