This might be a difference between the US and UK?

Preschools in the UK have curriculums they have to follow. That includes maths, reading and writing too.

I’m not going to comment on preschools in your country, but in the UK the kids who attended preschool are IN GENERAL the stronger students, socially, emotionally, and academically, when it comes to starting infants/ elementary school. Particularly in the less affluent areas. Though there might be some selection bias here too due to the kinds of parents who can sand their child to daycare verses those who cannot.

In the less affluent areas, I'd expect children not attending daycare to just not be getting anything at home. Presumably their parents are both working and cannot afford daycare. In the more affluent areas, I'd expect children only don't attend daycare if their parents prioritize their children over their jobs, and so they'd be getting much more positive attention than in a daycare. But, of course, we'd have to see a study differentiated by socioeconomic status to see what is actually the case.