Black has a lot of options and the positions can be a slower build up before any fighting.
So the theory seen during practice will not be as useful because there are so many options and transpositions. The strategic themes will not be useful or even noticeable to beginners.
Tactics open games have more straightforward themes and tactics. In these openings (like 1.e4 e5) beginners will also learn the value of tempo, development, and king safety more easily because neglecting them is more often a blunder.
Can someone tell me why are openings such as English, Reti considered unsound for beginners?