I think it's just got to do with the nature of the guitar, and the lack of formal education of guitarists.

Here's what the search for an F5 chord brings up. If you notice, there's quite a few of them that make the same noise, but have completely different way of playing them. The way a guitar is set up, if you play an open string, you can make the exact same sound by playing the 5th fret (4th, on one of the strings) on the string below it. For example, to play the A note, you can either play the 5th fret on the E string, or just play the open A string; they both make the same sound (It's actually used as a rough method of tuning, called 5th Fret Tuning). That's where tableture comes in. Not only does it tell you what chord to play, it also tells you what fingering to use. This can be helpful for starting guitarists, because they aren't likely to really know what an F5 chord is by name, or different ways to play it. It also shows, usually, the easiest way to play a piece, because they will usually try to keep similar structure to make it easier to change from chord to chord. Sheet music really doesn't show this, unless it's specified in the writing (based on my limited knowledge of sheet music).

Guitarists are taught tableture, not sheet music. If you learn a song online, it's in tab form. If you have a teacher, he's probably not going to give you sheet music. For people that don't have strong musical knowledge, it's a lot easier to connect with. With sheet music, you have to know how to read standard notation, as well as how to play what it's telling you. Tabs just say "put your fingers here" (however they don't usually give rhythm, but many already know the song, so they can figure it out), which is easier for beginners. Some eventually will learn how to do sheet music, but many just keep to tabs, because they're easier to them, and they're used to it.

Also, I think this is relevant here.