As per public statements, Google has ~100 million lines of Python code. Also, Google has used Python very heavily from the start, meaning that some of that code was written in Python 1.x or early 2.x days, before the language had packages, before standard modules like 'logging' or 'unittest', even before booleans(!).

It's just a lot of code to migrate. It takes time.

Disclaimer: Work at Google, wrote some of that code.