>>988267>So then why don't mountain bikes have wider drop bars?I'll get to that.
>the more upright riding positionBikes are fitted instruments. The riding position in the primary (only for flatbars) prosition is exactly the same if they're fitted for the same rider. Drop bars do not give you a longer/lower riding position if the bike is properly fitted. And if it isn't, well then the opposite might as well happen.
>Forward extension is irrelevantIf you look at early mountainbike geometry development you'll see this it patently not true. Early mountinbikes did just this to improve stability and steering control: they added lots and lots and lots of stem. 140-150mm was not unheard of.
This affords you two things. One being better leverage, and the other shifting your center of mass into the turn, stabilizing you.
This was all good and well and led to small frames with short wheel bases and narrow bars for ducking between the trees on the singletrack.
The downside to this short wheelbase and forward center of gravity is that it throws you over the bars when going down hill. So mountainbikes of late, as they evolved towards downhill stability, climbing boulders and ever more unforgiving terrain, have grown longer. Much, much longer. And slacker, with greater trail. A modern XC bike and course is to an early mountainbike what a heavy AM bike is to XC. All in the name of making them safer down the mountain. The rider has been pushed back, towards a more upright position to keep the center of gravity back, and stems are now super short out of necessity. This diminishes control, and the only way to gain it back is widening the bars and manualy shifting your upper body around, reaching to hold onto the opposite end of the bar in extreme cases.
The tradeoff is generaly thought to be worthwhile, but your assertions are both false and betray a lack of understanding as to how mountainbikes evolved, and why it has nothing to do with steering geometry.
