The reason may be the better space usage within the icon badge. Having square, or rounded square, you can place a bigger icon inside, with less space wasted:
The dashed line represents the placeholder for an icon and app name within the interface. The badge itself within this space can be bigger if it's square-ish than when it's round. This gives more space to use.
At the same time, the image/icon on the badge is even bigger (however, there are also icons that are round, that would almost perfectly fit the round badge - however this puts a requirement for the icon to be round only - in any other case it needs to be shrunk or cut - you can see it even in the example you have used). Having the icon big is important for keeping its details and making it more recognizable for the user.
There are, however, interfaces with round or almost round icons, however. One of the examples may be Nokia: