She very short answer is: because we haven't written one.
The slightly longer answer is: because we have a mobile version of the site, that you can access from your mobile... obviously, and it does pretty much all the things we think you'd want to be able to do from a mobile.
What would you expect to see from an app that our mobile view doesn't provide?
Apps are overrated. You need at a minimum one for desktop machines (we'll postulate it's a Java web start app so as not to have to write for Windows, Linux, BSD, Apple, etc.) plus one for IOS and one for Android, all sucking resources from the main development effort. They often run less than optimally for a given device's screen size and resolution. It's hard enough making mobile websites work well.
RSS is a very useful way to stay on top of things, but don't forget that we can also email notices when a forum has had a message added or a response to a message thread you've participated in. This is my normal mode of operation, in fact.
Some people, when well-known sources tell them that fire will burn them, don't put their hands in the fire.
Some people, being skeptical, will put their hands in the fire, get burned, and learn not to put their hands in the fire.
And some people, believing that they know better than well-known sources, will claim it's a lie, put their hands in the fire, and continue to scream it's a lie even as their hands burn down to charred stumps.