I love how that innocent question reveals how a popular phrase for objects (i.e. "guitar tuner" instead of "violin tuner") alters public perception. It hides that fact that other stringed instruments like harps/violins do use outside aids (electronics, smartphone apps, or websites[1].)

Some observations:

- guitars are more common than violins, so the tuning object that's more common is ... the "guitar tuner"

- other stringed instrument musicians like harp and violin players also use so-called "guitar tuners" or metronomes that generates a reference pitch A440. Plenty of Youtube tutorials showing harpists/violinists how to use guitar tuners.

- formal vs informal cultural background of the instrument: the "orchestral" instruments like violin have a more formal structure (school ensembles) so you often end up in a situation where a student violinist has no idea how to tune their instrument because "my music teacher always did it for me". Likewise, piano players pay "piano tuners" to tune their instruments. On the other hand, guitars are more of a "folk" instrument (home playing, campfires, self-taught, etc) so guitar players have more a DIY attitude for tuning from the start.

[1] https://violinspiration.com/free-online-violin-tuner-with-mi...