Why? Some bikes are easy and some not. As long as it is an easy reach there is nothing unsafe about it. When I rode up to the Arctic Circle, dismounting was a bit of a PITA because of the cargo I was carrying. All three of us refueled astride the bike whenever we could. That's my DL650 below and the Yukon River in the background. And that was one fast moving river. If one fell off the dock into the river, he'd be around the bend before anyone could thrown a rope.
And my friend Dan and his R100PD. He's ridding the the Arctic and north seven times up to Inuvek on the McKenzie Delta. He's also ridden to Panama on a 250 Sherpa. We never had an incident refueling without dismounting and I've never had a few fire, even when I overflowed a tank, standing beside it, and dumped it all over the tank and engine. I can see better down into the tank while sitting on the bike and looking directly into the fuel spout, with the exception of the Touring.