A side kick to the knee is a million times more effective than one to the head
For combat, that's a fair point. Even if kicking to the knee weren't more effective, it's certainly easier to learn and easier to execute -- this means that the ratio of Difficulty to Effectiveness is very favorable for knee kicks.
That having been said, if we're only practicing stuff that's "easy to do but very effective" then there are things that are even more effective that require even less practice.