system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the modern guitar come to be? Why aren't there any guitars with strings designated for each note?
gpt
>Why aren't there any guitars with strings designated for each note?

Because at that stage it stops being a guitar and becomes a harp. The important part of the guitar is the fingerboard, and an important aspect of being a guitar is that it's organised into spaced by frets -- the little wires you see going across the fingerboard at various intervals. Other string instruments can be fretless, such as the violin or the double bass, and violinists and double-bassists have to be much more precise about where they put their fingers; that's why chords on these instruments tend to be less common than they are on guitar. Other string instruments, such as the Indian sitar, have movable frets, which means you can change how the strings behave in relation to one another and to themselves.

Those frets are wider at the bottom than at the top because of the mathematical basis of music: it's all about ratios. At the bottom of the string (up towards the top of the neck of the guitar) the spaces are wider than they are at the bottom because the ratio changes. This is analogous to the way the strings of a harp change as you get closer to the player, from the long low strings at arm's length to the short, high strings right next to the harpist's nose. 

Being able to isolate a few different strings in different positions you can make pretty much any chord imaginable, and you also get lots of different options for making the same chord. That gives the guitar some of its trademark features; chords and versatility.