Not just that they don't want to "show off", but even more that they don't think it will be useful.
It takes a non zero amount of work to document a project such that it can be used and extended by someone who didn't write it.
I get your point that messy code may be useful for other purposes, but believe that the vast majority of users are looking for code to solve their problems, not "training data" or research.
That said, as you say, rights holders can do what they will.
Do you happen to have any evidence of that ever being done even once in the history of mankind?
It's a nice idea, but you should know by now the world isn't a slave to your desires.
I don't have evidence of this approach actually being used to inform language or tool design, but it's not like it's an outrageous concept that nobody's thought of before. Your hostility is perplexing.
The only one I can think of is id software, but they spent some time to clean it up I think. Also their products are games and already very popular.