Google engineers always choose the hardest route to solve problems. Why wouldn’t they? If your products are going to be shutdown in a few years anyway, might as well have a glowing resume from working on those products (resume-driven development).
Think about it, every time Google made a product with sensible tech stacks, those products were actually started outside Google and later bought by Google (Android, YouTube, etc). If Google made Android from scratch, there is no way they’ll use java and Linux, they’ll invent a new language and made their own kernel instead (just like fuchsia os which might be canned soon).