Short answer, MONEY. Dealers will order their stock without toppers. Then if you want them, they will charge you twice as much or more for them than you would pay the factory to install them.
Our Open range did not have them. We called the biggest dealer in our area to see what it would cost for the four slides. $3800. A few months later we had a leak issue and wanted to take it to the factory to get it fixed. When we dropped it off we asked about adding slide toppers while they had it. Cost us $1800.
A fifth wheel can have pretty much any option that a class A can. I am on my second fiver and both have slide toppers. I don't understand your statement that they don't have them. Fivers can also have auto leveling, washer/dryer, dish washer, powered shades, fireplaces, etc.