Well, they do have the fastest supercomputer in the world currently and it's made with homegrown chips. No Intel ME backdoors there. Smaller chinese companies could, for a little more money, get similar performance buying 8x V100 machines from NVidia. I don't think they want to share their advancements in AI fighter pilots with USA. They have a big lead.

What is the hardest thing to accomplish with something like a TPU? Is it the IP or the fabrication?

How does the TPU design offer improved performance? By leveraging IP or fabrication improvements?

Scale. Even if you design the fastest chip, you need to convince people to use your proprietary solution that has 0 users currently. Until then you don't have enough volume and scale.

Becomes a chicken-and-egg problem.

Google was able to solve it by designing TPUs for internal usage first, therefore reaching minimum scale, and then making changes and offering it publicly.

So you'd need to be:

1) Chip designer

2) Massive internal user

3) Cloud service provider

I don't see any other company that has all 3. Amazon has (3), and maybe (1) if they hired the right people. Microsoft could have enough (2) to justify it, but they're going the way of FPGAs.

Why is Google investing in its own?

Cost advantage.

Bitmain already announced some.