For the same reason downhill racers don't go with no poles in the Olympic Downhill, and the same reason Seth doesn't drop into an Alaskan heavyweight line with no poles:
Because even if you might not need to use them 90% percent of the time, you are still a better, more balanced skier with poles, and the pros don't want to do something that could potentially handicap their ability to recover if they lose their balance.
Yes, I know, you can do laps through the park with no poles and feel great etc., and if everything goes well then you could do a slopestyle run just as well with no poles, as you could with poles. BUT, if things get a little sketchy, having poles is a more stable option to recover with.
Wanna test that theory, go ski moguls both with and without poles, see which version gives you a quicker recovery when your balance gets compromised.
No poles can be fun, and it is also a great tool for gaining balance and working on grabs, but at the end of the day, using poles is simply a more effective high performance setup than not using them.
