Why is lexical-binding's global value ignored?

[abq]

If somebody does (setq-default lexical-binding t), the intention is 
obvious. But in current versions of Emacs, when loading a file that 
doesn't specify a local value for lexical-binding, the global value is 
ignored. Why?

IOW, what would break if the global value were honored? It defaults to 
nil, which is already how files are loaded if no local value is 
specified, so honoring the global wouldn't change anything by default.