We had a law. The Citizens United ruling by the Supreme Court invalidated it saying the First Amendment takes precedence so we’d need another Amendment limiting speech in politics, which is complicated. I believe we already have laws about foreign money in politics, but we have extremely weak enforcement for it (and weak enforcement of other political laws in general). If we made stronger laws requiring PACs to report where all of their funding came from the current Supreme Court would likely knock it down saying anonymous speech is also protected by the First Amendment.
Of course those asshats did. They don’t care about the law or the constitution, just their feels.