Maybe it's because they're afraid of having to retune between every song on stage. But if you throw some creativity into the mix, you can write several songs in the same tuning just by moving the capo up and down the neck.
The most frequented alternate tuning is called Drop D (just drop both E strings down to D ... making your guitar strings spell DADGBD). This tuning basically puts you in G major. Strum it open. Experiment with your typical chord positions and notice suddenly the chords are thicker. This works really well with bar chords, and even better with fingerpicking.
Several folks take it a step further and tune the B string down to an A (DADGAD) - putting you in D major.
Other tunings I particularly enjoy are GGCGCE, or better yet CGCGCE. DADF#AD is fun too. One of Joni Mitchell's more interesting tunings is from Song to a Seagull, where she uses BF#BBF#B. Another frequently recurring tuning for her is CGDFCE.
Ani Difranco has some really bizarre tunings like AADGAD (the low A tuned to A below the usual E!) for Dilate. Also EEBABD. I've also heard that she's strung bass guitar strings on the lower portion of her acoustic to obtain lower notes.
In other words, go nuts! Think of standard tuning as a suggestion. If you're afraid of standing on stage with the audience doe-eyed while you try retuning your whole guitar, then try alternating the tuning by changing only one string. Once you open up to this new world of alternate tunings, even standard tuning can start to feel like a new frontier!