It's a carry over from before deregulation, but all the major carriers have continued to abide by it. You can get transferred to another airline if they can get you there more quickly than your original delayed or cancelled flight.
Also, while Global Entry automatically give your access to TSA Pre-Check, you don't have to enroll in Global Entry to get TSA Pre-Check. You can enroll in that program for free if you have status with most of the major carriers. TSA Pre-Check is for domestic flights. Global Entry for returning through customs for international.
I can't speak for whether Global Entry members are more likely to get the express security than just the regular TSA Pre-Check people though. I imagine the Global Entry background checks are more in depth.