It's just that treadmill running is just so darned boring. What is there to look at while you are running on the treadmill? The TV?
Quite often I lose myself in my runs because the scenery is so captivating. Where I live and run, the scenery isn't so spectacular, but year round, Mother Nature comes through with rewards almost every morning. For the last couple of years, I've attempted to capture what I see on my runs.