People just don't make mass changes to existing working code. Mostly they cannot. Even if the tooling was available, which it's not, it's also about reeducating their developers, who don't want to or can't change. Plus it'd have to be recertified. It's all cost with no benefit.

Except, allegedly, at Google. But is there any evidence they actually do this, eg. in public code bases? Or is it just hype?