additional gates may not even be available for lease, and the cost is probably prohibitive to justify these instances of "arriving early".
same thing happens on pretty much every airline at LAX, for instance. i've also had it with DL at ATL and JFK, and UA at ORD and SFO. it's aggravating, but honestly I'd rather wait 15-20 minutes and arrive at a gate than have to use the air-stairs and a bus.