what is reasons of disabled shell_exec() on cpanel code example

Example: error exec() is disabled for the security reason laravel

There is a php.ini directive called disable_functions. Functions added to this 
list will be disabled by PHP and when you try to execute those functions, you 
get this error. As mentioned, in all probability your hosting provider has 
added exec to the disabled list. This is a common practice in shared hosting. 
  
You will need a dedicated server if you really want to run exec 
(or some hosting provider who provides pseudo-exec functionality). It is a bad 
idea to trust a shared hosting provider who allows you to run exec unrestrained.