You would, but FB and some other companies are dealing with 20 years and millions of lines of code written by a small army of developers. If they decide to switch language and rebuild their services, just doing it manually will in all likelihood take longer and more people if they don't apply tricks like automatic conversion, because it takes more effort to rebuild than to build fresh due to multiple reasons.
But the options aren’t “manual” and “ai”. There is a third option of using classical static analysis and compiler techniques that have been around for decades to do transpilation and large scale refactors.
There are entire companies already build around this.
There's also a fourth option: do nothing. Leave the existing services in the languages that they're written in rather than translating them into other languages and laboriously debugging the resulting code. If these services communicate through well-defined APIs, any new services, which can be built in new languages, can communicate with the legacy services.
It may be more cost effective to keep around some developers who know the old code base and the language it's written in.
