I know what you mean but I don’t think there’s enough churn in the top 40 these days. i.e. songs stay around in the top 40 longer as there’s not as many new songs coming in to replace them for whatever reason. If stations played the top 40 only it would be stagnant for months.
It’s not like previous decades (60s, 70s, 80s) where new music was constantly being added. And good new music at that.