People have already tried and failed doing exactly that. Because they aren’t required to report their trades when they’re making them; There’s a significant gap in reporting, so by the time the trade gets reported the stock has already increased/decreased like the congressperson expected.

For instance, let’s say [Generic Congressperson] sees a bill that will help a particular company. They know this bill is going to pass. So they buy stock in the company, vote for the bill, and the stock goes up. They then sell the stock. Three months later, those trades get reported. And now it’s too late, because the company’s stock has already been boosted. So buying now would just be buying after the boost.