Why do some cms do not delete the installation file, but require it to be deleted manually?

Question

CMS

Why do some cms do not automatically delete the installation file, but require you to delete it manually?

Answer 1

Because often the script does not have sufficient rights to delete files from its directory.
Not without reason, many CMS recommend setting permissions 0777 (rwxrwxrwx) to the upload directory to upload files via the web interface.
For example, if php is executed under Apache from the user apache:apache, and CMS was uploaded via FTP by Vasya Pupkin under the user vasya:wheel.
It is also possible that scripts loaded by the user vasya are executed on his behalf.
In this case, the script can delete files and directories without additional gestures.

Answer 2

While the file is being executed, it is not read/write.
In addition, the installation may not go as planned. Returning the installation file (and in some cases the entire directory) is redundant.
PS Don't use CMS :)