BITD mtb handlebar width, with mostly straight bars available, was benchmarked as shoulder width or a bit more. As time has gone on handlebar width has grown well beyond that as popular to compliment changes in overall geometry.
As this occurred it became evident that placing your hands further apart did the red thing illustrated above and so angled bars became a thing to alleviate the problem. Lots of available options today.
Personally, as I ride both mtb upright and road drop, I still prefer a narrower flat bar albeit more than shoulder width. Using straight grips, tried the ergo kind but they did nothing for me, and my bar height about 2”es below Saddle height works best. For me.
The drop bars I use are shoulder width but they afford more hand position options that I toggle through while riding. Most long range riders seem to prefer drop bars for what it’s worth.
At the end of the day whatever makes your positioning on the bike most comfortable is the best option as you are more apt to ride a comfortable bike than one that causes issues. As the miles add up though one may feel the need to do adjustments to your position on the bike but don’t feel like you have to do what everybody else does, just what works best for your body.