Because they wanted to demonstrate the shou sugi ban technique.
BTW - I first saw the technique demonstrated sever yrs ago on Fixer Upper. Joanna did the exterior siding on one of their projects....,she also burned them to have an all black finish
I'm sure they went into it with the burned woodgrain showing in mind as the desired finish - held one up against the ombre and said nope, doesn't work - and at that point pivoted to, just do them in all black and be done with it
Personally, I like the shou sugi ban w/woodgrain showing the most on beams, posts/columns, o