Currently Being ModeratedJul 1, 2013 5:48 PM (in response to xin_li97)
Might be that fact that you are talking about two different operating systems.
iMac = OS X
iPad = iOS
The Age of Empire III is available from Amazon > http://www.amazon.com/Age-Of-Empires-III-Mac/dp/B000HZRLIM
But the requirements are for a very outdated Mac operating system. v10.3 which would not run on a newer iMac.
Currently Being ModeratedJul 2, 2013 7:01 PM (in response to xin_li97)
It's probably worth noting that getting on the Mac App Store requires extensive security/certificate signing work, which some developers forgo in order to: cater to those who do not have/use Gatekeeper on OSX Mountain Lion, increase availability, or most importantly, just not deal with Apple's rules. There are many, many apps that do this: take for example Adobe's Creative Suite/Cloud.
TL:DR; the only way to officially install iPad apps is through the App Store. So in order to make a product available for iPad, it must go through the App Store process. This is not the case for OSX, since there are many ways to legally install applications.