At least why not in modern orchestras? There is an argument that says it is too quiet so the rest of the orchestra will down out the guitar. However, that is true if we are talking about adding one guitar player in a set where there is multiple violists, cellists, etc. For example having 4 guitarists playing chords will definitely be loud enough in an orchestra.
On a side note, ancestors of guitar are sometimes part of baroque pieces.
There certainly isn't one single reason, and, yes, “that's just how it's always been” is probably an important point.
However, I would quite agree that guitars should not be part of a standard orchestra: for the same reason pianos aren't, and shouldn't be, part of an orchestra.
Orchestral music takes a lot of its attractiveness from the art to weave individual, expressive melodies into a whole that's a whole lot more than the individual melodies. Themes keep wandering through the various instrument sections, and each permutation sounds different but can be used to introduce counterpoint in an analogous way. There aren't really any accompaniment instruments as such.
For this to work properly,
Acoustic guitar pretty much fails in (2.) and, due to limited tone control, also has trouble with (3.). Electric guitar is risky in (1.) and (2.), and phrasing and intonation tends to come out pretty different from orchestral instruments – a problem you also have with piano. So these instruments don't really blend into the orchestra.
Of course that doesn't mean you can't use guitars or piano with an orchestra.
But neither of these options is really in the spirit of symphony, with its, as it were, “communist” hierarchy.
The argument for why it isn't now is pretty much because it has not been done that way. Orchestras are typically structured quite traditionally.
Your point around volume is definitely valid - a guitar is too quiet. You'll also find that 4 guitarists are too quiet unless they are amplified - the attack when a string is hit may shine through, but the remainder will be drowned out by strings, brass or woodwind. As a good example of guitars working with an orchestra, see Deep Purple or Metallica for a couple of examples - amplification, and a lot of it!
I suspect the main reason though is tradition. The vast majority of orchestras historically did not use guitars, so why should they now?
A classical guitar (or two or three) could be a lovely component of a chamber ensemble. All that remains is to write/arrange some music for Guitar and Strings. You will notice that in concertos genrally, the ensemble often leaves "room" for the featured instrument, either because the composer wants to give emphasis to the musical theme (Mozart's piano concertos) or because the feature instrument is softer than the ensemble (eg harpsichord). So get writing! I'll come to the premier.
Guitars, like pianos, are percussive instruments. As such, they are severely limited in being able to contribute to a smooth sound texture. They can be employed as solo instrument or in chamber ensembles (baroque and renaissance ensembles routinely employed lutes and there was more similarity in sound, volume, use and instrumentation between the lute and the viol family than we have now between guitar and bowed strings).
For percussive instruments with a strong attack, employing a player group is not expedient since they need to be perfectly in sync all the time or you get a pattering of attacks on every note. You can see this problem in pizzicato passages of a regular orchestra: they take considerable practice and focus and yet there almost invariably still is some fuzziness left in the result.
I personally think that the guitar is not "sophisticated" as well for being in an orchestra. A classical orchestra is a very relaxed, sophisticated scene where you see more violins. The guitar, on the other hand, is more enthusiastic.
