But the chip is maxxed out, is it not?
No. The 5090 dies is using less than the max available specs for the full die utilization. This is the die wafer yield binning thing paying into the specs table. Few dies that come off a completed wafer are perfect. When 75%+ have flaws you have to do something with the imperfect dies. They sorted by functionality and quality of functionality and grouped into product class. The 5090 dies are near perfect but not quite. A full die would have(if I've done the counts right) 24,000 CUDA cores, 180 RT cores, 720 Tensor cores, 720 TMUs, and 192 ROPs instead of the 21,760 CUDA cores, 170 RT cores, 680 Tensor cores, 680 TMUs, and 176 ROPs is has. A 5090ti would have much more enabled and viable.
So to give you the short answer, it's not a maxxed out die, there is room for improvement so long as TSMC can refine the process enough to make more perfect wafers.