It's not simple. You can't just 'turn it on' when some people may have 2000 friends -- the absolute constant barrage of friends notifications would be maddening and that amount of nonsense could easily cause other issues [there's potentially one hell of a lot of communication going on when you have each person telling 2k other people every time they're on off, times all the people online]. So now you also need to add friend 'groups' so that players can manage which groups they get notifications for. But now you also need to add group management and related features/gui - people would want to have friends in multiple groups, etc, etc. And if you're going to do all that work for groups, then maybe it's worth considering implementing some sort of OS level 'clan' system while you're at it?
And on and on. People love to assume everything is 'easy' in the dev world. That's almost never the case. Even a 'simple' change in the store, for example, is a fairly major task given you're touching mission critical software.
