I assume because they use the t.co link to verify items have been clicked for their ad services, but I can't be sure. Either way, as I user I couldn't stand that wasted time, so I went about trying to fix it.
My point is that the app can record that click asynchronously. They have full control of everything in the app, and it bothers me in the first world problems section of my brain that they haven't tackled a lot of low hanging fruit for speeding up user interactions.