With an app you get exactly the opposite. An app would have to be a hub for all your Discourse-forum-based activity, and would have to be somewhat generic, like the standard version of Tapatalk.
Tapatalk does have a “custom” offering — but at that point you’re getting a customized app that has to be distributed by each individual forum - this is a whole additional avenue of maintenance and support (also, do people really want 5 apps for 5 different forums?)… That just seems crazy to me when the alternative is simply visiting an already existing site, which has also been customized.
I think “Why isn’t there a native app?” is a question that can’t be asked without first answering “What benefits would a native app provide?”
And what’s the answer to that question? It would certainly create a lot more overhead and maintenance work… and for benefits? push notifications? some offline capabilities?