I understand the appeal of chromatic mountain dulcimer
General mountain dulcimer or music discussions
I don't think it's so much that it sounds like a guitar. The logic is basically "If you want to play a fully chromatic instrument, why don't you just play an instrument like a guitar (or banjo or mandolin, etc.) that is already chromatic?"
The purists would say that the diatonic fret pattern is a defining feature of the dulcimer. As you begin to change one of the dulcimer's defining characteristics, the instrument is moving away from being a dulcimer and transitioning into a hybrid instrument. Not a guitar exactly, but beginning to look and play more like a guitar and less like a dulcimer.
To get the sound of a guitar, you would need to increase the size of the dulcimer's soundbox, extend the neck/fretboard beyond the soundbox, increase the number of strings, and adjust the gauges of the strings. Each of these changes is a movement in the direction of the guitar and away from the dulcimer.
If the only change made is to fret the dulcimer chromatically instead of diatonically, then the instrument will still sound more like a dulcimer. However, it will have some of the playing features of the guitar.
Some Common Features of Each Instrument:
Dulcimer = Smaller Soundbox, Diatonic Fretboard, Fretboard Does Not Extend Beyond Soundbox, Fewer Strings, Lighter Gauge Strings, Played on Lap with Fretboard Facing Up
Guitar = Larger Soundbox, Chromatic Fretboard, Fretboard/Neck Extends Beyond the Soundbox, More Strings, Heavier Gauge Strings, Played with Hand Reaching Under and Around the Neck
Putting a chromatic fretboard on a dulcimer body without changing any other features results in a chromatic dulcimer. Not exactly a guitar, but a step in that direction.
Incidentally, there is nothing wrong with playing an instrument modified from its traditional form. Likewise, there is also nothing wrong with preferring to keep the instrument in its purest, most traditional form. To each their own. What you play and how you play it are decisions to be made by each individual. Basically, if you enjoy playing it, that's all that's necessary.