It's really not, the players run set routes v the same players all game long, they make slight adjustments and that's it.
In the NHL you have literally millions more variables, offsides, icings, odd man rushes, sizing up different opponents as there is actual heterogeneous opponents i.e 6'9 260lb guys and 5'7 150lb guys, sticks, skates and pucks flying and getting hit way harder due to the speed.
And the coaches/qbs aren't holding your hand doing your thinking for you.
Is hockey hard? I don't know, you tell me. We need to have the strength and power of a football player, the stamina of a marathon runner, and the concentration of a brain surgeon. But we need to put all this together while moving at high speeds on a cold and slippery surface while 5 other guys use clubs to try and kill us. Oh yeah, did I mention that this whole time we're standing on blades 1/8 of an inch thick? Is ice hockey hard? I don't know, you tell me. Next question.
That quote just begins to summarize the difficulty of Hockey.