In addition to bcbrown's comment, trust/security is a factor. Neighbors may allow their kids to play in the hallway or go down to the lobby and check the mail o

In addition to bcbrown's comment, trust/security is a factor. Neighbors may allow their kids to play in the hallway or go down to the lobby and check the mail on their own. But if there's a sense that the neighbors aren't known, then parents may feel uneasy about it.