Why `system` is not obscured by these PHP obfuscators

I recently tried to select an obfuscator, I tried to google it:

https://www.gaijin.at/de/tools/php-obfuscator
http://www.pipsomania.com/best_php_obfuscator.do
https://www.tools4nerds.com/online-tools/php-obfuscator

They work well to obfuscate variable names and function names, but none of them can hide the function systemCan someone tell me why that is?