Why can't we use yum?

Plesk uses an RPM-based installation of Apache and PHP. cPanel does not. cPanel compiles Apache and PHP from source. That is why you cannot use yum to install or update Apache and PHP components on cPanel. RPM packages will break the source-built Apache and PHP on cPanel.

 

Actually you can also use pecl utility to install php modules in a simple way, as this options does not needs to compile modules manually.

 

That is correct, but PECL is very different from yum. PECL actually does compile PHP components from source, but since they are extensions, they do not require recompiling the entire PHP code base. The same is true for PEAR.

yum is an update manager for rpm (Red Hat Package Manager) files, which are collections of pre-compiled binary files. PEAR and PECL can be used to install some PHP extensions, but it is not the same as using yum or rpm, which is what the original question was about.