It's targeting a different market. The MDN web docs are excellent reference material, but if you're learning or early in your career they're impenetrable. W3Schools has a reputation of over-simplification (to the point of being wrong sometimes), but that's not at odds with being useful to people at a particular stage in their career.
For another example, I gather that most of the high school chemistry I learnt is wrong, but it was mostly pretty useful in understanding the physical world as far as I needed to progress in my education at that age. W3Schools can be useful in that respect.
Education is a series of smaller lies. It's one of the corollaries to "all models are, wrong some are useful."
Some lies are actually better than the truth. My favorite example is that shooters are taught to imagine their sights being in line with their gun and that's they're shooting horizontally. But they're not, they're shooting in an arc so over short distances bullet just rises a little. Don't ask why. Why it's cool is that people using this model are better shots.
For another example, I gather that most of the high school chemistry I learnt is wrong, but it was mostly pretty useful in understanding the physical world as far as I needed to progress in my education at that age. W3Schools can be useful in that respect.