Poor children attending preschool doesn't necessarily mean that schooling is the answer. It could be that the parents who would send their kids to preschool (regardless of socioeconomic background) are the kind of parents who give their kids the skills needed to succeed later in life. There are tons of confounding variables when it comes to stuff like this, so the jump from "preschool is a good predictor of life success" to "we should invest more in preschools" isn't necessarily a logical one. Correlation does not imply causation, and all that.