What's the scoop on "scoops"?
Instruments- discuss specific features, luthiers, instrument problems & questions
Which is why some older dulcimer ancestors had their frets right along the edge of the instrument- because either they had no raised fretboard, or one that was less than 1/2" tall off the top of the instrument. Having the frets run along the edge of the body enabled using a noter when the frets were on or immediately above the soundboard itself. Imagine trying to use a noter on frets that are installed down the middle of the soundboard, directly into the instrument top rather than on a raised fretboard.
One of the very definitions of the mountain dulcimer specifies it having a "raised fretboard running down the center of the instrument". Many instruments that are earlier dulcimer ancestors have frets along one edge of the instrument- epinettes, hummels, langspils...
As to strum hollows and 'sweet spots', it's true that the mellowest sound of all will come from plucking/strumming a string near the halfway suspension point. Yet violinists, guitarists, banjoists, etc don't generally bow/strum/pick in the middle between nut and bridge. Some oldtime banjo players play "up the neck" to get a particular soft mellow effect, but bluegrass, tenor, classic, and Irish banjo players play near the bridge purposely because they like the crisp snappier sound. The strings are more rigid to play on right next to the bridge, with almost no flex. So most folks feel more comfortable plucking/strumming several inches away from the bridge, where the strings begin to have more give. On the other hand, bowing at the halfway point creates just too much string bouncing and flexing, thus a bow is usually kept pretty close to the bridge where the string tension is stiffer. Another physical issue is that the halfway point is where the seventh fret octave is, and people do a LOT of fretting in that area- you can't be both fretting and strumming or picking in the same area at once. (I'm assuming folks define the 'sweet spot' not as exactly the halfway point, but rather as somewhere between halfway and the bridge.)