Because that opens the door to huge privacy issues. Expand your thoughts on this and you will see all that implies to do age verification, and how that makes it possible to other entities to track down who is using the App Store.If it's Apple's platform, then why does Apple not want to handle age verification?
Parents should be responsible to keep their children safe. Most parents nowadays are very lazy about this, don't make their lives any easier, or they will keep forgetting their child is in the back seat of their car.