2 major things about this:
First off, especially when it comes to solos, almost nobody's live shows are "proper studio performances." I don't think you're understanding the intricacy that goes into it, especially with the harmonics he uses. There's just not really a physically possible way to duplicate the layering he uses on some parts in one sitting by 1 person.
I've seen him come close-- fretting with not just his left hand but the top finger or 2 on his right hand while strumming with his thumb and ring/pinky fingers, but, well, that shit's just ridiculous. Was for the solo on hard lines, sunken cheeks iirc. Youtube really sucks for finding good pantera clips ; ;
Anyway, point being it's just the raw physical limits of what he's performing, and not just with him. 1 guitar, minimal allowance for effects changes, etc. Even listening to Jimmy Page at his very best live, you'll notice the same things where he just can't quite fill it in as perfectly as sitting in a studio.
Secondly, what makes dimebag so, sooo amazing wasn't really his soloing, anyway. It was kind of like John Bonham with drums-- sure, he'll always be known for some crazy solos and whatnot. But when it comes down to it, 30 years from now, it's Dimebag's riffs, just like it's Bonham's rhythm, that will really be remembered.
And on that aspect, the dude is fucking amazing, and his live performances are like studio quality... and it's rather ridiculous. Extremely few people can play such a defined quality live, let alone have that talent and the creativity to have given a style basis for like nearly every metal band since. Vinnie is absurd in his ability to basically put down a studio performance live as well.
Here's a very simple example of why Pantera held up as long as they did, and some reasoning as to why:
First off, the riff is fairly simple. The solo is also made slightly less intense than the album version, as pretty much every guitarist ever except Jimi has done, for the sake of keeping the sound quality reasonable without proper production and in 1 take on the fly.
However, this allows him to just keep it absolutely perfect. Take out the background noise, Phil's random additions, and put in a little production for sound quality, and this might as well be a studio recording by any above average metal band.
http://www.youtube.com/watch?v=87uEuhMj92k
Here's another one (song starts like a minute in):
Any self-respecting metal fan will instantly recognize this riff instantly, and it'll probably be that way for another 50 years. Sure, other people might be able to play it, just fine. But when it comes to Dimebag's part (and even the solo in this recording), and hell, Vinnie's for that matter, you'll never see them done better... let alone live, in one sitting.
http://www.youtube.com/watch?v=Hh51rtXOAAo