I think there are a lot of ways to get out of your comfort zone that don't involve kids. Failing to find them is either laziness or a lack of imagination.
The thing about having kids is that once you make the leap, you don't get to step back. A single decision to drop the birth control for a bit, and bam -- you're a parent with daily responsibilities for at least 18 years. That'll certainly get you out of your comfort zone, permanently.
But other things you can do are more or less entirely voluntary, and you can stop doing them at any time. So it takes more willpower to maintain, even if doing those things is strictly less work than raising a child.
The thing about having kids is that once you make the leap, you don't get to step back. A single decision to drop the birth control for a bit, and bam -- you're a parent with daily responsibilities for at least 18 years. That'll certainly get you out of your comfort zone, permanently.
But other things you can do are more or less entirely voluntary, and you can stop doing them at any time. So it takes more willpower to maintain, even if doing those things is strictly less work than raising a child.