A lot of guitarists have no problem with playing open tunings because they can pretty seamlessly transcribe the notes along one string to the whole tuning, like how the 2nd fret of the low D string in open D is an E note so thus a finger across the whole fret is an "E chord".

It's once you get into alternate tunings that are outside the "open tuning" norm that it becomes, at least for me, memorizing finger patterns and getting fine coordination locked in. The theory gets thrown out for "what sounds good" and "hmm that sounded about right".