Because it's not made to accelerate training, just inference. The TPU is an 8-bit fixed point processor less power hungry than GPUs, so it won't help research, only deployment for large projects, running in the cloud.

Why use it, if it locks you in?

If you're a hobbyist, why would you care about lock in?

I care about being able to carry on doing my hobby stuff 5 or 10 years from now. With NVidia I can at least be confident that as long as my graphics card keeps working (which feels like something under my control, unlike Google shutting down their products) I can keep running my code on it.

Because you don't have control over what Google does. They may kill TPU altogether leaving your work irrelevant.

You don't have control over what nvidia, intel or the rest of them either.

If you want to get your 'feet wet', then why bother?

Because they have client facing products and backward compatibility is important to clients.

Hobbies come, hobbies go.