Look at the financials of EA and Activision. I'm seeing razor thin margins for both. Keep in mind too that both companies pull in ~$4B of revenue annually, an estimated six to seven times what Zynga pulls in.
Of course, it's not all about margins. The long bet is that Zynga will continue to grow its top line. But the margins, the free cash flow - investors notice. They notice that FarmVille didn't take $100M to make like WoW, or many years and >$100M to make like Star Wars: The Old Republic. Zynga's games are lean, the rewards vast, and the margins high. Trust me - investors notice. To wrongfully paraphrase Obi-Wan, "these are margins you are looking for."