Another issue is that guitars are often played in a amplified live setting. You want to be able to adjust your tuning while the rest of the band is playing or other noise is present, and you don't want the audience or others to hear you tuning (so you can't hear it yourself either).

I can tune my guitar relatively but I can not for the life of me tune it with from an external reference pitch like a piano key. I use a tuner if I need to be in tune with other instruments.