Because it would display the nightly rate as $X.
But then at checkout, it would add in "house cleaning fees" etc (which I don't dispute is a fair fee to include) but it at times can grossly misrepresent what your true nightly cost is when searching.
Maybe this will be a step in the direction like Telco's have had to do with creating simplified & standardized "nutrition labels" for pricing.
Not including fees in the nightly rate makes sense as some are fixed rate, and having the option to see the cost for the total stay (including fees) solves the problem.
It can be done. Marriott show the total including taxes. Mind you, this was as a result of a legal settlement, so they get no credit.
If you search for an area without dates, it comes up with some arbitrary dates and applies the fees and displays per night cost accordingly.
So it's technically possible. They just don't want to.
At least on booking.com it depends on what settings you have.
We all know it is a scam fee. People wouldn't be as mad over a hold on some amount (usually returned) insuring the host against an egregious sloppy mess.