Because it's the way the vast majority of free-to-play, IAP supported mobile games work. It's been a long time since I've played a free-to-play mobile game which doesn't require an internet connection and even a lot of the "premium" mobile games out these days require an internet connection. I guess it's possibly something to do with the expectation that mobile phones and a lot of tablets have a built in internet connection? Dunno..
Mostly games seem to require require it for stat tracking, in-game currency handling, leaderboards, levelling up, in-game item purchases (due to the list being stored on servers to allow new items to be added without requiring a full game update), IAP purchases, achievements and so on.Cloud connectivity and all that.