It is an absolutely massive amount of work to turn something designed for your custom software stack and data centers (custom rack designs, water cooling, etc)

It is an absolutely massive amount of work to turn something designed for your custom software stack and data centers (custom rack designs, water cooling, etc) into a COTS product that is plug-and-play; not just technically but also things like sales, support, etc. You are introducing a massive amount of new problems to solve and pay for. And the in-house designs like TPUs (or Meta's accelerators) are cost effective in part because they don't do that stuff at all. They would not be as cheap per unit of work if they had to also pay off all that other stuff. They also have had a very strong demand for TPUs internally which takes priority over GCP.