How are you defining "everything" and "upfront"? Upfront as in the first page shown after searching?
Because many, many airlines/car rental sites have a complex muti-step process of different fares, extras etc until you get to the final stage
Sure you will have upsells but if a price for a service is presented, that should be a final price. You can't tack on "resort fees", the price presented must be inclusive of all the required charges. For example as much as I dislike Booking.com, the price they show for a room includes everything — tax, mandatory cleaning fee and city tax if applicable.
Also, so much is unbundled these days, you have to be really careful what that initial price really includes. For example, with Sixt, they often don't include the basic CDW + Theft coverage which for a long time was always included in the base price. I assumed it was law in most of Europe. Luckily Avis, Hertz, Europcar don't stoop that low
You're right - you can book a car, and if you don't inadvertently agree to extras either verbally or on the ipads at the rental desk, and don't incur any extra charges/fees during the rental, the price you pay should be what you initially reserved :-)
And you're right about booking.com - they seem to do a fairly good at at incorporating tourist taxes etc into the final price.