At major events and dedicated music clubs the dj needs to have technical ability too. The crowd have paid 10-30 to get in an expect quality.
Sure I could play an hour set in a major club and have great tune after tune but if any of my beats clang that wouldn't go down well. Let alone mixing multiple tracks in and not even dropping them before bringing in the next full track.
Andy c mixing is insane. Grandmaster flash I've seen twice and the tricks he pulls is madness, that is not just track selection. It is ability to tear the roof off.