I know exactly why I myself sort of have to look at the fretboard when i play dulcimer ... when I play banjo, not only are my fingertips pressing on the fretboard, but my left hand or at least my thumb, is curled around the neck and thus giving me a physical reference point , a sense of where 'home' is for frets 1-5. Much the same as when folks plant a pinky when flatpicking across strings on mando, guitar, bluegrass banjo, and dulcimer too. Without that pinky plant, it's much harder to pick individual strings accurately when not looking- the pinky tip is the reference point. even with clawhammer banjo, most folks at least have their thumb on the back of the neck and it works well as a reference point. I can play banjo pretty well without fretboard looking, but not dulcimer.
With a mtn dulcimer, we don't have our left hand wrapping or curling around the neck for a reference point, once you go up and down the neck either fingering notes or using a noter, it's hard to know where the frets exactly are anymore. Of course this is a skill that can be developed and improved (look at highly skilled hammered dulcimer players), but most folks use the eyes becasue the physical reference point is not there as it is with necked instruments.
I know I have a terrible time when we play at night while camping, if we are playing by low lantern light and my fretboard is not getting any light at all.
A good exercise is to try playingh for varying time lengths with your eyes closed....quite an education!
All that said, I'll look at my fretboard if I please to! :)

I think many people still look at the fretboard because even though they may have been playing for years, that amounts to very few hours overall when they belong to a club that meets once a month for a copuple hours, and they don't practice in between.
women ! (tongue in cheek, honest