The issue is in committing to 3.g3 a move earlier. Then if Black plays 3...d5, White must play the Catalan (with 3.Nf3, he still had the option to play a different line in the Queen's Gambit). Or take 3...c5 - against 3.Nf3, White has some edge in a few different lines. But against 3.g3 Black should be pretty close to equal, since as @RemcoGerlich mentioned, the Fianchetto variation isn't White's best try against the Benoni.
Of course, some people still play 3.g3 anyway, since they're fine playing with a fianchetto in all these lines. I think it's what Avrukh recommends in his books. It's just not everyone's cup of tea.