For it to "even out" they'd only have to increase your reach ~50%.
They do way more than that. And they give you an inherent legitimacy that putting it on your own site doesn't. It's not just handling refunds; it's the certainty as an end user that you'll get one hassle free.
Without Steam (or another retailer with similar traits), selling an indie game would be closer to a pipe dream than really hard. In almost all cases (and this seems to apply even to AAA publishers as most of them come back), the 30% they're taking is money you wouldn't have without them.