Because it sounds utterly ridiculous when grown-ups use her arguments:
Sen. Paul is basically reading from Atlas Shrugged. And it's nonsense, as Sen. Sanders demonstrates. Further, I think Paul knows it is.
If you're just tuning in, Ayn Rand believed (as apparently Rand Paul believes) that taxes were only taken by force, and were therefore always illegitimate. She believed that a government levying taxes and providing services from those taxes was doing so "at the point of a gun," even if nearly everyone in the society agreed to the taxes and services.
It's a seductive argument. Of course governments force you to pay taxes—though in the U.S., it's unlikely that the local police will break down your door and haul you off to jail if you don't. But the piece that Rand's argument misses is blindingly obvious: there really isn't any way to ensure that everyone contributes without some sanctions for failing to comply. Otherwise people would simply not pay taxes.
No, it isn't the force that makes taxes illegitimate to the Rands and Pauls of the world. They just hate taxes. In Rand's vision, we wouldn't have governments; private interests would provide everything we needed because the "market" would encourage them to do so. For example, if there were enough demand for nuclear submarines, a company would enter the market and make them as long as doing so were profitable. Same with voting booths, bus service to poor neighborhoods, and firefighting services.
It turns out, there was a time when most things our government supplies came from private interests. We call this time "feudalism," which no doubt Rand Paul would like to see return to the world.