It is weird, but it was done for good reason.
Right now, the check of "hey, should this account earn interest on it's balance" only occurs every time your balance actually changes. To make interest pay automatically, they would have to do that check constantly throughout the day, or during some sort of batch processing once a day. These processes if done automatically would consume resources on the machines running the blockchain.
So in order to optimize the blockchain slightly, they only check when funds are transferred.
Someday we'll probably see automatic interest, but for now, we need services like this to make it happen.
that is interesting to learn. i was just thinking with my wife to use steem as a savings platform. But now i am quite confused.
How is interested calculated if not by the amount in an account?
It is calculated based on the amount in the account itself, it's just not automatically updated every few seconds. I'm not sure I followed exactly with what you were asking :)