@Tudor29 It isn't displayed as a separate line item to be helpful to the guest, it's displayed that way because that is the only way it can be displayed.
I suppose a guest could just be shown the total of the nightly rate, the taxes, the Airbnb service and the cleaning fee once they have entered their dates and number of guests, but I should think a lot if guests would want to see the breakdown, even if you don't.
If you are talking about seeing a nightly price in search which includes the cleaning fee, before you have entered your requested dates and the number of people, that would be misleading, because as Branka and Silvia explained, it's a one time fee for a booking, regardless of whether you stay 1 night or 10.
Lets say a listing is $100/ night with a $50 cleaning fee. If the host listed the house at $150, you would think it would cost you $450 to stay 3 nights. But it wouldn't- it would cost 3x $100 + $50= $350.
Does that make sense?