MIMF

A kid asked me the question, and I did't want to say 'That's the way it is' because many kids don't understand or accept that statement (and wisely so).

Well, why not?

Tradition. Violas da gamba do have frets, traditionally tied, but only for half the fretboard.

Frets make only equal temperament possible. (At least, only one temperament, it can be slightly different from equal.) Fretless instruments can play in any temperament whatsoever. If you for example play with a harpsichord tuned in meantone, you'll soon see the advantages.

Tradition is the best answer. Also intonation, temperament, sliding (portamento used to be a big part of violin playing, as big as vibrato is now).

Also check here: http://www.frettedfiddle.com/

Best modern conjecture is that Viola da gamba and violin were parallely developed during the mid to late 15c, the VdG being fretted and aimed for amateurs; Violins unfretted for professionals. Much more difficult to take advantage of unfretted instrument to mess with the temperament and intonation; also more challenging to deal with the wider stetches on an instrument tuned in fifths (violin) as opposed to fourths and one third (VdG).

I offered my mandolin to a violin player to play once & the reply was that "the frets mess me up"--