Okay here is my list.
#1. Safety. See the posts and photos above, or go with your own. The Balsa wood and staples building approach of the overwhelming majority of RVs is just not worth even a small fraction of what they cost us, and they are DANGEROUS!
#2. Flexibility in layout. I can build EXACTLY what I want / need for myself and my family, not get stuck with and make do with what someone else thinks is a good idea.
#3. Financial flexibility. I can piece a Skoolie together on my timeline, within my budget restrictions.
#4. Envrionmental impact. I'm no tree hugger by any means, but I despise how wasteful of resources factory built RVs are. They end up in a land fill rather rapidly, and are a poor use of resources. For what it's worth though. An older rot case RV might just be a great way to source up the expensive major appliances for a Skoolie build if they are still in good shape!
#5. I am an avid DIYer. And I don't trust profit seeking corporations to build with the sort of effort and quality I would put into my own stuff. It just wouldn't be cost effective. For example, doors and drawer fronts in my cabinet builds have been solid wood raised panels with mortise and tenon joinery. Drawer boxes are half blind dovetails etc... I can guarantee unless you are paying in the going on a million USD, you aren't going to find that in a factory built RV...
#6. I can use what I have if need be. Instead of dedicating a new stove / oven to a build, I can use an old propane camp stove. I don't HAVE to build in a black water tank / toilet, I can use a Port o Potty that has the ability to mount down to use as a cassette toilet etc...