In my opinion, it always bothered me that the phone speaker at the top is not louder and the default speaker for audio. if the bottom, or back, main speakers was moved to where the phone speaker was then it would solve the problem of muffled media and it would probably add some good directional sound
A safety concern I think. The last thing you want is to be blasted by a sudden LOUD notification sound or a tune while holding the thing to your ear, as if you were in a call. It could be done, but would probably have to ensure it can't play loud if the proximity sensor was covered, e.g. holding it to your ear. Which I think could have its own problem, like if you were using a case with flap that covers the proximity sensor, it wouldn't play loud.
And if they didn't do something like that, manufacturers might be facing multi-million dollar lawsuits from customers who allegedly deafened themselves with their phones.
EDIT
I had an analogue brick dumb-phone back in the early 90s, NEC 9A, and I'm sure that did use the top ear speaker as the ringer, which was the only loud noise that phone could make, no messages, no notifications, no alarms, no music.
But having said all that,
there is a new smart-phone that does have front speaker at the top and bottom and is intended to be loud, and we've discussed at length here on AF. The Marshall London.
http://www.engadget.com/2015/07/16/marshall-london-music-phone/
"The loudest phone on earth"...according to Marshall.