You think if someone is better than you they must be 'hard core' and play all the time.
Not at all.
but that hand eye co-ordination to play video games well? Not everyone has it, a lot of people don't really.
Again making the 'average' player pretty mediocre.
The game sales, isn't from casual / occasional gamers. It's from average gamers.
Average gamers don't master games. They play them, and move on. Half Live 2 was a pretty easy game, and pretty top rated... according to steam ~14% have finished it, meaning 86% of players did not.
Even if we filter out the people who barely tried it (got it free or bundled and played for 15 minutes then moved on) we can look at the people who got halfwaty through vs the ones that finished it, and again its approximately half. Or duke nukem forever ... 25% completed the single player game on EASY. 1% completed the game on 'insane'. Sure, around 25% didn't get much past the title screen... but even filtering that out, a shockinly low number of people who played it beat it. Spelunky ... same thing... most owners haven't beaten the first stage. But even of the ones who have, only a fraction of those have gotten very much further.
That 1% who beat duke nukem on insane. Those are the type of people who dominate in multiplayer games. They are the ones who memorize all the maps, master all the weapons, and plug away at the SAME game for days and weeks on end. They are not 'average' they are the 1%.
The average gamer ('gamer' IS 'casual' for any given title. They just churn through a lot more games. Where a truly casual player will only play games a few times month AT ALL. The average GAMER only puts 10-40 hours into a game, if that, before moving onto the next one. They are 'gamers' because they spend a lot of time playing games, they are much better than a truly casual player at most games, but they are still just noobs compared to the the hardcore players in any given game.. The hardcore players have racked up 100s even 1000s of hours in their chosen mastered titles. Me, I've got 75 hours in left 4 dead 2, I more than hold my own and am an above average player. But compared to a hard core player, I don't know squat. And for left 4 dead... I'm pretty casual... even if I play games 30-40 hours a week... I only put in a session of left 4 dead once or twice a month when the right friends are online and mood takes us.
But my brother in law, with nearly 800 hours in the game, knows where every short cut is, every possible item spawn point, every respawn closet, every wall that breaks during a horde, routes to kite tanks on every map, proven tactics on where to set up for every finale. I've got another friend with a similar number of hours in both of Payday 2 and Infestation, he's another very good FPS player, and yet he's almost a liability in Left4Dead with only 5 or so hours in it. and he's got a group he plays with regularly and that takes it to a whole other level. To get thrown up against him and his friends in a multiplayer battle (even if there's just 2 of them) ... the average gamer has absolutely no chance at all.