While Feige does (thankfully) see them from a very artistic standpoint, Iger sees these movies as nothing but dollar signs. The goal is to make as much as possible. Sure, Deadpool actually did make a lot of money... but by going R-rated you limit your audience. Not all R-rated movies will see Deadpool level success, so PG-13 it is for the MCU.
Not to mention with this Universe now in it’s 11th year, more risqué characters would seem out of place in a world of PG-13 fare.
I do have to say that I think that, of the MCU franchises, GotG would probably be the one I’d most like to see as an R-rated movie. Especially with James Gunn on the script, it could be great.