Why is Extra Attack in the Attack Action?

@john_Doe995 Why is Extra Attack in the Attack Action definition, yet the only Action limited to “On your Turn” (Reserved for non-Actions)..? it prevents multiple attacks with an opportunity attack

— Mike Mearls (@mikemearls) January 25, 2016

@john_Doe995 I was trying to understand why it doesn’t work with Ready Action – while AoO isn’t the attack action. same basic deal – readying is meant to have some built in cost, such as with casting

— Mike Mearls (@mikemearls) January 25, 2016