Ask HN: What's the drawback of a one-time purchase?

I've worked in SaaS for the better part of a decade, and the decade before that was desktop software/game engines. After watching these companies work for a bit, I look at the churn stats and think -- man, you could just charge a single one-time fee and make almost twice as much; then every year or so, just charge for new features to capture anyone still using the product and wanting the new features.

Further, recurring revenue has a ceiling, almost entirely dependent on churn (simple math and easy to see when graphed out). With one-time purchases, the only ceiling is how good you are at finding new/repeat customers, you don't have to worry about keeping any customers at all.

So, why do we focus on recurring revenue models?