Why is RX 580 and 590 same memory clocks?

[tigersty1e]

The RX 480, 580, 590 all have the same memory clocks. Why is that? The 480 came out long ago. By now, I would've thought they could've increased memory speeds.

Does the chip not need the extra memory increase?

 

[jpiniero]

The 9 gbps GDDR5 is too expensive I reckon.