15 cents is huge. It's 15% of the way to the next note. Even 5 cents sounds noticeably out of tune. Trained musicians can easily tune to less than 2 cents without using tricks like beating to get even more accurate tuning.
Guitarists may be out of tune, but chances are more likely you're hearing a poorly intonated guitar. You can tune the open strings perfectly, but if your string scale-length deviates from what your fretboard expects, you'll have notes that progressively get more out of tune the further down the neck you play. You can't correct this with tuning, you need to adjust the tensions in your bridge saddles, and most amateur guitarists are afraid to do this.
Also you mention live settings, it depends on how big the group is I guess, but at smaller venues and smaller bands the stage monitoring is often so bad you can't hear your own guitar.
Guitarists may be out of tune, but chances are more likely you're hearing a poorly intonated guitar. You can tune the open strings perfectly, but if your string scale-length deviates from what your fretboard expects, you'll have notes that progressively get more out of tune the further down the neck you play. You can't correct this with tuning, you need to adjust the tensions in your bridge saddles, and most amateur guitarists are afraid to do this.
Also you mention live settings, it depends on how big the group is I guess, but at smaller venues and smaller bands the stage monitoring is often so bad you can't hear your own guitar.