Shooters, single-player shooters especially, condition us to reload as a Pavlovian response to getting a kill. Resisting that urge is one of the first things most new CS players have to unlearn. Most weapons in CS:GO take between two and four seconds to reload: plenty of time for someone to peek around a corner or move through the space you should be defending. But maybe more importantly, in close-quarters, reloading betrays your position with sound. Consider the trade-off: would you rather have a full mag and an enemy who knows where you are, or a shallow mag and an enemy who's out of position?
Don't underestimate how much you can do with four or five bullets. With a rifle, it takes three or four non-headshots to take an enemy from full health to none—and in late-game situations, it's even less likely that the enemy around the corner has 100 HP.