Adored TV made a great video explaining Nvidia's decision on 8nm and why the performance/watt is so bad.
View: https://www.youtube.com/watch?v=tXb-8feWoOE
Basically as you know, the power curve is not linear, so pushing higher clock speeds means exponentially more power is required once you go past the efficiency part of the curve. Nvidia knew 8nm clock speeds were never going to be good, 1400mhz base and 1700mhz boost is really the best they could expect for a mass produced card. So instead, since 8nm wafers are cheaper, they decided to make huge dies and rely on higher transistor count to give more performance.