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Facebook actually does some compile-time optimizations, but also ships some of the profiling data for JITing between early deploy phases and the rest of the fleet: https://engineering.fb.com/2021/03/03/developer-tools/hhvm-j...

Also, Facebook used to do ahead of time compilation (HPHPc) but eventually HHVM managed to outperform it.

JIT compilation has the opportunity to do profile-guided optimization at runtime. JIT compilation is also simpler when distributing an application to non-identical servers, as it can optimize for the exact hardware it is running on.

HHVM's predecessor at FB was an AOT PHP to C++ transpiler called HPHPc. There are a lot more optimization opportunities with JIT compilation when dealing with dynamically typed languages like PHP.

Because they first tried to AOT, and a 2nd project with a JIT managed to get better performance out of it, with improved development workflow.