Maybe because parents and home environment matter as much, if not more, then teachers?

Middle-class and upper-class parents who work 9-5 jobs have the time to spend playing and teaching their kids on nights and weekends.

They have the money to buy good, nutrious food.

They have the knowledge (either from their own, self motivated research, or from their friends who do the research for them) to do things like limit screen time and not use a TV as a baby sitter.

Poor people can do all this too (and many try and do), but being poor makes it harder, which is one of the reasons economic class is inherited.