Answer: Because Steve said "No."
Not trying to be flippant but this is the reality of Apple products. There isn't always a logical reason for the inclusion/removal of what most of us would consider "natural" features. But Steve and Co. decided to do thing a certain way and you can rationalize it all you want or not. They don't really care what you think. Buy their products as-is, fully subscribing to their vision and design or not. You have a choice and by choosing Apple you are effectively outsourcing your right to an opinion other than theirs.
That's why Android is so popular. There are many users who simply can't abide by giving up their opinions as to how their phone should work/operate. Its also one reason (along with piracy and theft of service) why Jailbreaking is so popular. In a weird karmic way, Steve Jobs and Apple have become "The Man" that he was so anti-establishment towards when building his empire. Microsoft has long since lost the suit of conformity and given way to Apple Conformity.
When it comes to phones, I'm happy to give up my right to an opinion. I like how iOS works, I buy their products.
But wondering and/or complaining about it is useless. Your opinion means nothing to Apple. Just your money.