These games are inherently competitive with an unbelievably steep learning curve, so people git gud, which drives off casual players, repeat ad nauseam. The only people who regularly play CS are either absolute masochists, or very good at clicking heads even if by that game's standard they're only silver or gold or whatever.

To be halfway decent at CS means that no singleplayer shooter will ever be a challenge to you again, because there's no intersection between casual gaming and competitive shooters.