Why are so many guitarists self-taught?

We're probably seeing the influence of the cultural mystique of the guitar. When we look at popular culture, we see there are no "French-horn hero" video games and you don't see people performing "air violin" while blasting their favorite classical music. Thanks to rock music's cult of personality and the fact that most popular musicians play the guitar, no other instrument really has the same exciting aura of artful sexuality. This results in tremendous motivation for learning, and with the guitars relatively low barrier to entry (unlike say, the piano or drums) and the image of the rebellious, maverick guitar-slinger, self-learning can seem almost preferable to formal training. I think that if any other instrument enjoyed the same cultural role, you would see self-teaching of that instrument bearing similar fruit.

I've heard that some self-taught guitar virtuosos have said that formal music training would only stifle their creativity, and there are obviously plenty who have been quite successful without such training, but I feel like music theory is just a tool that can potentially help, but not hurt one's ability.

I think there is a degree to which the guitar is eaiser to self teach - it sits on your lap and the technique is quite natural - I play violin and until someone taught me I could not figure out to hold the instrument let alone play it.
Also we have this conception that someone is somehow less a guitarist if they are not self taught - because Kirk Hammet took lessons from Joe Satch we think him less a legend. I don't think knowing much sheet music is needful unless you play jazz or classical but most music theory can be self taught without taking lessons or exams.

Because all you need to learn is a couple chord shapes and you have the tools to write any number of songs. In this day especially with sites like this one, youtube, tablature sites (tablature makes it easier than learning another instrument right there) all the tools you need to learn to play are available online - for free!

Well, i am a self taught guitarist & artist myself. I rate myself as a mediocre player but i feel that really the inherent creativity & a desire to play peacefully is somewhat curbed by many of the Guitar trainers- i have observed this in my counterparts who had been given proffesional training. The problem with many(and not all) tutors is that they treat teaching guitar as teaching mathematics where you have to be correct in technique right from the moment you begin it. They often don't often emphasize on their pupils really enjoying the sanctity of the instrument. Being self taught, i had the essential liberty of practising whenever i felt like practicing and not when someone asked me to.