Because that would be extremely complex and expensive to do for a negligible effect.
It would also make the economics of streaming viable for the majority of artists rather than the small pool of folk raking in the bulk of the money.
revenue * 0.7 = total payout total payout / no. of streams = payout per stream payout per stream * streams of an artist = payout for artist
In the proposed system you'd have to calculate the payout per stream on a per-account basis and have additional complexities like how to handle family accounts etc.
b) As I said, the effect for the vast majority of artists would be quite the opposite of negligible.