The obvious answer is that performance/watt is literally everything when talking about laptops. Most people favor laptops these days even if they (like me) leave their machines "docked" 90% of the time.

    do we know for a fact that a 90w Apple GPU would 
    perform 4x faster if it was running at 500w

It's somewhat besides the point IMO, but don't GPU workloads tend to scale well simply by adding more compute units? This should be pretty easy to extrapolate by comparing M1 / M1 Pro / M1 Max power consumption and performance relative to the # of GPU cores.

I would certainly like to see Apple go nuts and make a true, cost-and-power-consumption-are-no-object world destroying GPU. If only as a halo product. But I wouldn't hold my breath.