isshinryuronin
Senior Master
This is an ideal some schools of thought may strive for, but I think it is mostly just that, an ideal. I would say it's a result of stressing the "do" of karate-do, rather than the more realistic tactics of karate-jutsu. By this I mean that the philosophical approach of perfection of technique and spirit (courage, commitment, etc.) is the main goal to lead to self-development, thus striving for the perfectly timed and executed punch.Many Karate systems emphasize "drop opponent by one punch".
This outlook is typical of Shotokan, a style mostly developed for this purpose in the school system. This is an admirable goal and develops some good attributes in the practitioner. But, like a one-punch knockout in boxing, it's impressive, but hardly the norm in actual combat.
More often, a series of moves are needed to set up and lead to the big punch, especially against a mobile experienced opponent. Feints, sweeps, movement, traps, grabs, checks and tactics to move the opponent out of position are often needed to open the opponent to a strike. Not to mention counter punching after/during blocking, as opposed to a clean "stop hit."
Both the do and jutsu approaches have their value - it just depends on the practitioner's individual goal. (Maybe "sport" karate lies in-between them, somewhere.) They are mostly, but not completely, mutually exclusive, IMO. I have switched to a "one punch" Shotokan style attack as a surprise change-up in sparring and it's quite effective used that way, catching the opponent off-guard. But I still view them as two different approaches to the art of karate.