I still maintain that just sticking 650B on the sidewalls makes you go faster.
I think history and trends comes into this a bit. There will be a lot of 26 bikes out there that just came with narrower bars because that’s how it was a few years ago. And it wasn’t until around the same time as 650B happened that the whole trail bike and wider bars started to catch on.
A 2013 bike I bought (and recently sold) came with 660mm bars out of the box and 100mm stem. I upgraded it to a 50mm stem and 760mm bar and it was a lot better for it. Much more control.
My last of the 26 inch 160 ‘duro bikes came with 780mm out the box and it feels perfect for that kind of riding but I have 760mm bars on a 26 inch 140 trail bike and that feels perfect too.
So for me bar width goes with travel and not wheel size since all my hoops are 26 inch.