Conversely, whenever I see people talking about server-side JS, I can't find any reason why I wouldn't use PHP instead.
PHP has a vastly simpler toolchain (making it much more effective for rapid iteration), much more consistent and well-thought-out syntax, a more extensive standard library, type safety without having to transpile code from another language (so no build processes that rival C++ in complexity just to still have interpreted code at the end), native and full-featured object orientation, generally better runtime performance, and a package ecosystem with Composer that isn't overrun with inane vanity projects and supply-chain vulnerabilities.
The only major downside to PHP is that it's not great at multithreading, but if you're building microservices where parallelization is handled by an external orchestrator, then you can design around that pretty effectively.
PHP has a vastly simpler toolchain (making it much more effective for rapid iteration), much more consistent and well-thought-out syntax, a more extensive standard library, type safety without having to transpile code from another language (so no build processes that rival C++ in complexity just to still have interpreted code at the end), native and full-featured object orientation, generally better runtime performance, and a package ecosystem with Composer that isn't overrun with inane vanity projects and supply-chain vulnerabilities.
The only major downside to PHP is that it's not great at multithreading, but if you're building microservices where parallelization is handled by an external orchestrator, then you can design around that pretty effectively.