This.
Perhaps that blank slate design is all that is necessary.
By using such a basic design it means any variety of individuals can find a way to adapt the product for their own specific needs. Which would explain the ridiculous amount of third party support for iPad for niche users to the mass market.
There's a reason why the KISS principle was so widely accepted, and is still applicable 50+ years later.
That's the thought that I had as well. Companies will always look to preserve margin so if the majority of users are adding cases for protection then the inclusion of a kickstand could be seen by design/marketing teams as a waste of margin. That margin can't be recovered from accessory sales if a significant amount of cases are sold by third parties.
I get why OP prefers a kickstand and I get use out of the one that is on the logitech case that I have on my iPad Pro but clearly if I was looking for additional protection on the non-kickstand portion of a surface tablet I'd still have to add weight/thickness to it with a case. In the end Apple's decision to just go with a simple flat back and allow for greater flexibility of case designs probably boils down to just margin and giving users the flexibility to decide what they want on their own.