Because there are probably only ~200 PL's with truly significant usage, and they tend to have a variety of purposes and be used in historically different contexts -- much like constitutions.
The same arguments that programmers have over declarative vs imperative is much like the arguments political scientists have over parliamentary vs presidential. Static vs dynamic type checking is much like civil law vs common law.
And there's similarly nothing "scientific" in proving that one programming language is "better" than another, but people have strong opinions, and experience will tell you which ones are good ideas to pick for a project and which ones are not.
By the way, I'm not sure you know what a case study is -- it's a report written by a person. Nobody's written billions of those. And you don't do comparative analysis on case studies -- a comparative analysis is a case study, where the cases are countries (or programming languages).
It's incomparable to PLs, which have perhaps billions of case studies on which comparative analysis can be done.