Why CI4 Cannot Run in a Subdirectory?

Hi.

I've read somewhere that CI4 is not supposed to be run in a subdirectory (like Laravel?). What is the reason for this behavior?

In my previous experience, when I created a website for a client using CI3 or Wordpress, some clients usually want the staging version is uploaded to a subdomain subfolder (like example.com/staging/). What is the solution to this?

Thanks.