Alot of people who go to MO schools are not religious and in fact many people who go to MO shuls like Young Israels and the like are not reglious either, but they are Pro-relgious. Meaning they like the religion but have limits.
The reason more Charedi dont leave is because they are more forced to stay. If someone wants to leave MO they can do it and there isnt much penalty, however in the Charedi world women are married as teenagers and frequently have children before they are 20 and the men are also married quite young and there are children as well.
When there are spouces and young children its much harder to leave. Also MO families dont usually shun OTD kids where Charedim (especially Chassidic do) so if you leave you are really on your own.
MO’s can also function in regular society so its not so hard to leave, if you are charedi you really cant function too easily and blend it