He probably considers battling the arrancar beneath him.
Everyone is already engaged in battle.
The Espada all have opponents, and their fraccion are all defeated.
Yamamoto has actually done something, he has trapped Aizen, Ichimaru and Tosen, and defeated Allon.
True. At this point, the old man is probably waiting for his fire-trap to fail, so he can engage Aizen. He can't risk being held up in a fight with the Espada when Aizen joins the fray. Glasses-man of Doom may not look like much, but the Espada obey him for good reason. Against an opponent like that, you just can't afford to split your attention.
Yamamoto knows this. He is, after all, 2000+ years old and he has been in a lot of battles. Lady Tactics is, as they say, his bitch. The Captain-General is probably thinking out battle plans or some such.
Or maybe he's just a bastard who likes to torment his subordinates and call it a 'useful character-building experience'.