In recording/studio you always (well, mostly) have the best possible acoustics in the mix and recording rooms. You simply do not have that convenience at live venues where the acoustics and bass trapping range from barely acceptable to complete crap. Not to mention every venue you go to has entirely different acoustics and frequency spikes/drops. That's why live mixing is completely different than studio mixing. Also, in a studio you have a rough idea what you usually should cut/boost/HPF/LPF from each instrument, but you can't tell at all in different live settings. Not to mention once the crowd comes in, you will have a completely different frequency range due to humans being both a diffusor and absorber. Depending on how many people are there and where they stand/sit, you will have
completely different high frequencies.
Basically you could compare it to trying to mix a song in a studio while the room you are in changes shape constantly and somebody is moving a master EQ HPF slider around all the time to random places. Oh and each musician you recorded keeps turning their own fader up. Yeah, and every instrument leaks to every mic you have on. And finally, the crowd is constantly drowning your monitoring out by screaming and a state official is ready to shut down the place if you go beyond 85 dB RMS (the crowd is still yelling at 95 - 100 dB)
Fun, eh?