There are plenty of AntiVirus apps available for Android while it is claimed that antivirus apps are not required for ios. Following are few reasons to support it.
The Android Market as Google Play, allows any submitted application. Apps are not strictly checked or pre-screened, they're simply submitted, and anyone can download them. Different types of app can be submitted to the Play store, even one that interacts with core operating system. Developer can create app with malicious activity, which can steals data, disables phone, or can perform other harmful operations on device. When people complain about app, then Google take notice and remove an app.
Apps submitted to the iOS App Store have to go through a strict scanning process.
Apple have testing teams who manually inspect each app and make sure that each app is without bugs and does not perform malicious activities on iOS device. Apps generally run in a sand box environment and cannot make any major changes to the way the device works.
Since all apps are pre-checked before they are posted on the App Store, there is almost no chance of an application being malicious.
Some Apps on Android takes permissions which are actually not necessary for their functionality. They usually gather personal information from user device and store it on remote server. While, iOS apps are properly checked to ensure that they ask for permissions required for their working.
Apple denies existence/possibility of malware for their platforms and doesn't allow AntiVirus apps into their app store