She has other advantages that make her physically appealing. Besides, it's not like she's that flat. Her opponents are so enthralled with how her front looks exclusively (and without trying to look for any evidence in the manga) that they fail to hold any other female characters to the same standards that they judge Sakura by. I can promise you that there are plenty others who are even "flatter" than Sakura is. Want some names? Karin, Karui, Kurotsuchi, Fu, Shizune (in spite of what she might say otherwise in the anime, it definitely appears so), and Tayuya... Those are the ones that immediately spring to mind, but there are probably others out there. If Sakura is considered flat then these other characters must be so frontally deficient that their chests have literally receded to make a cavity in their thorax.
Oh, I definitely know that. It's just haters logic to me, to be honest. Mention Sakura the most, but ignore all of the other girls who are pretty much the same as her when it comes to their chest sizes.