First, note that Airbnb shows it "built into the rates" when you put in the number of days you are staying when making a reservation. See this article from 2022: https://www.theverge.com/2022/11/7/23444561/airbnb-total-pri...

But to answer your question - because it is a fixed cost per stay.

If you stay two nights @ $250/night, you'll pay an additional $200 cleaning fee (so that would be $350 night if I were to "build cleaning in") If you stay 6 nights @ $250/night, you'll still only pay an additional $200 cleaning fee (so it would be $283/night if were to "build cleaning in").

Airbnb doesn't allow me to charge a different $ based on the number of nights stayed. But they are showing an all-inclusive cost to the user when the user is browsing with their # of days, so there is nothing hidden...

and to your "give it time" statement, it's been over 5 years...