It's not true all the time. A lot of participants from Chipmixer don't post much regularly. Some of them end up posting 5/10 posts per week while they could have posted 50 posts per week to earn 0.0375; that's why Chipmixer campaign is so generous & so does the participants.
Here are some stats of some of the Chipmixer participants which you may enjoy & will help you to change your thoughts.
Payrates highly correlated with quality of posts. At first stage, only highly qualitied users accepted in that campaign so it makes sense to see their posts stay at high quality levels. With them, making constructive and high quality posts are their favorite habits and I believe they feel shame of themselves if they make shitposts.
The correlation between post quality and payment rates is a two-sided one.