Because that feature of gmail is not a part of email, it's a part of gmail the product. And it is not "undoing" sending a sent email, it's cancelling an email that was never sent in the first place.
Because email doesn't work that way. Gmail doesn't send the email for a minute. It would be like your boss asking you to send this email and you wait a minute for him to change his mind before you presses send.
Why doesn't it count and why does it matter how Gmail works behind the scenes?