Because sexual response changes.
Something that felt fantastic yesterday may feel irritating today. The same person may prefer gentle stimulation early in arousal and much stronger stimulation later. A certain practice may work beautifully with one partner and feel completely wrong with another.
And perhaps the most inconvenient part for the idea of “sexual mastery”: sometimes the technique is perfectly fine, but the person is thinking about tomorrow’s deadline, anticipating pain, worrying about their body or simply unable to relax.
That is why claims about a “universal orgasm technique” always make me slightly suspicious.
Good technique really does matter. But actual sexual skill begins when someone can notice that the instruction manual no longer fits and not take their partner’s body personally for failing to follow it.