I think he specifically does not want you to know what he thinks about these things.
Is it really a secret? Calvin and Hobbes contains a ton of relevant commentary. Just the amount of time Calvin spends outside tells you something.
He probably would leave it as observation rather than presciption, like most humorists.
It's not secret, more like private. He quit because he didn't like being a celebrity and having people pry at his personal life.
Kids spending a lot of time outside was quite normal in the 1980s. There wasn't much to do inside. Even more so when Watterson was himself a child, which is probably what inspired a lot of his cartoons.
I think it also has to do with not wanting to accidentally ruin the legacy, like some of the successful tv series do with their last season.