From what I understand, and I could be very wrong, the fear is that directly attacking the Houthis will be seen as an escalation by Iran. The west doesn't want to get mired in yet another war in the Middle East, so instead they've adopted a defensive posture around their shipping lanes with Prosperity Guardian. This is also why Saudi Arabia's own navy vessels are conspicuously absent from the operation, despite the Houthis being a geographically prominent thorn in their side.