That's not limited to Lemmy. It's what happens across all social media. A tiny percentage of people will post, a larger percentage will comment, and the vast majority will lurk. The general numbers I've heard thrown around are 1-9-90, but the posters and commenters are probably much lower than 1% and 9%. Especially as platform size increases.
As for why, and this is hazarding a guess: posting takes effort, commenting takes less, and lurking takes the least.