I know it does, but why?
Just a teenage girl who loves playing guitar way too much, if that's even possible.

I live for my girlfriend. <3
As in a high end guitar (i.e. costs a lot of money) or as in the higher pitch notes?
- Strandberg OS 7
- A cheap fender strat knock-off not worth naming
- Garageband
- Boss GT-1
- Potato
Clarity and high end are mutually exclusive terms.
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I also have to do that. Cottaging this weekend
You are habitually confusing terms, I beg you, read up on what these things actually mean before asking
our ears are more sensitive to higher frequencies and will pick up on them more easily, so when you've got a lot of high frequency overtones in your guitar sound they tend to 'grab' your ears a bit more. But that's a totally different thing to clarity.

having the overtones too prominent without enough of the fundamental frequency present in the tone can lead to a sound that you hear/feel as nothing more than percussive when working with other musicians, because the actual notes you're playing won't really stand out, only the attack transient will - and probably not in a pleasing way, either. So that's an example of how high end/treble frequencies don't really add clarity.

It's all about finding the right balance to fit the situation, really - if you don't have enough high end, the tone will be muffled, but if you have too much, it's gonna be really shrill with a piercing attack and if you're working with a drummer you'll be fighting against the cymbals for space in the mix which is not a good thing.
I like analogue Solid State amps that make no effort to be "tube-like", and I'm proud of it...

...A little too proud, to be honest.
I guess I did use absolutely awful wording. I meant why do high harmonics (trebble) increase the (I can't think of a word other than clarity even if that's the wrong term)? Blompcube answered with what I wanted to hear. Thanks.
Just a teenage girl who loves playing guitar way too much, if that's even possible.

I live for my girlfriend. <3

It's what I've been calling "acousticness", a lot of high transients, and something I strive for. I don't think it cuts through especially well in a mix, as it adds an unnecessary level of tonal complexity.* However, that kind of tone is very satisfying for one who fingerpicked acoustics for a long time before taking up electric.

*In unamplified acoustics, the "simpler"-sounding dreads cut through better than the more complex "waisted" designs.
Although you didn't ask exactly what you meant to ask I think I can sort of answer what you were trying to ask.  I am going to over simplify this a bit because I don't want to spend all night writing about it.  Inside your ears you have something called a cochlea.  When we hear something it is because of little hairs inside the cochlea are being moved by the pressure caused by sound waves hitting your eardrum being passed through bones and eventually making their way to the hairs in the cochlea.  Hi frequency waves move fewer hairs than low frequency waves.  When you have fewer hairs being affected by the sound it is perceived as clearer tone.
Not taking any online orders.
Low frequencies require very little power for a power amplifier to produce, where as low frequency take a LOT.   Overdoing low end probably bogs down the amplifier a lot more while you can crank treble no problem.  Just a guess.  Plus, too much treble can sound ear piercing while lots of low end sounds kind of like a big slow explosion.  Might have something to do with wave length?  The wavelength of low frequencies is HUGE, so in one second you might only be experiencing 20 wave lengths (20 hz), where as on the treble side, you're experiencing that wavelength 20 thousands of times a second (20kz).  Maybe your brain is quicker to perceive the sound and hence it sounds quicker and sharper.  

Of course, this is all me just guessing with no real scientific studies to back it up. 
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I am going to over simplify this a bit... Blah blah blah... a cochlea. ...Blah blah blah... When we hear something it is because of little hairs inside the cochlea .

Thank god you didn't give the complicated explanation. Hairy cochlea indeed.
Okay, so here's the deal. We're most sensitive to sounds in the 2000Hz to 5000Hz range. We can discern these sounds at the lowest intensity.
To give you an idea how high (treble) these sounds are, the fifth fret on a high E string is A=440 Hz. Most guitar speakers can barely reproduce 4500Hz, so that will give you an idea where those sounds live. Therefore, if the notes you're hearing have good chunks of harmonics in those 2000-5000Hz frequencies, we hear those clearly.