Designing ICs is a very expensive treadmill. If you can buy fast enough hardware to do your job, it's really hard to justify spending a lot for something that may be nominally a little better on some metric.
There's commodity computing of comparable throughput that other automakers can incorporate without too much cost, but it doesn't fit into the existing TM3 vehicles because it has a larger footprint in power, space, and cooling than HW2.
Here, Tesla spends a bunch of money to develop HW3 and overcome this problem, but the advantage in a new car design between HW3's efficiency and commodity hardware is limited.
