Because while the green card itself has no minimum requirements on time spent in the US, with the exception of the DV program, all of the visas with green card pathways have one. So yes, there is - it's just attached to the visa pathway that determines green card eligibility, not the green card itself.

Can you think of any other reasons why you might have been downvoted? It seems a little conspiracy-minded to jump to “agenda pushing” I think.