Earlier quoted context omitted.
Perhaps some people care more about what would be convenient to them than whatever laziness or lock-in Adobe intended. Or maybe they believe that Photoshop might have better interchange with future versions of itself if its file format was sane.
What decade was Photoshop first written in and how long did it take to open a file?
Exactly. I clearly remember working on a 20MB PSD file in the mid 1990s (a restoration job done on a borrowed Mac of a badly damaged photo of my grandfather; the picture still sits on my mother's dressing table.) That took an absolute age to load and I know there wasn't much more RAM than that in the machine. It worked well enough though.
Old formats like PSD are better viewed as archaeological artefacts than as exemplars of some elegant ideal.