One large factor is that when somebody really dislike the result of a code generator, they most commonly cope by editing the generated code by hand. With other metaprogramming techniques people can't do the same, so they fix the issues.
The end result is that code generators are usually a set of mostly-functioning tools that create bad code that is almost, but not exactly impossible to understand but that you will be required to change at some point.
One large factor is that when somebody really dislike the result of a code generator, they most commonly cope by editing the generated code by hand. With other metaprogramming techniques people can't do the same, so they fix the issues.
The end result is that code generators are usually a set of mostly-functioning tools that create bad code that is almost, but not exactly impossible to understand but that you will be required to change at some point.