In App Email, why not in app texting?

I was just watching the 3.0 keynote and watched the in app email demo when Oracle is on stage.

What is stopping Apple from building this into the core OS? For instance, if you get a text, why can't you just text from in the app? If they have the ability to do in app email, I fail to see why in app text could not work.

Now, I know in-app email works by having it built into the app, but why won't apple just allow people to reply to texts from within an app and then go back to the app?

But seriously, what is stopping Apple from doing this? Does it not work well or something?

I really cannot figure out why they are not doing this. This would fix 90% of my frustration with the phone (which isn't much because I love the phone). I hate having to quite an app to go to the SMS (soon to be messages!) app!