It's because when someone suggests using one's imagination in this forum, it's usually as a response to someone suggesting that the gameplay could be improved.
Ex:
-It would be nice to have more engaging missions with some narrative aspects added to them.
-Use your imagination instead!
In that case, the person saying "use your imagination" is basically making the statement that imagination should be seen as a replacement to actual gameplay rather than an addition.
Isn't that a moot point though? In open world game, gameplay possibilities are endless. And thus it is physically impossible to develop such game that 'fulfils it's potential' that way. In such games, imagination *always* works for better enjoyment. Issue is, some people can let it go and can make their own stories in their heads, and some need to be spelled that out. Especially when they try to push themselves trough and not try to enjoy game for what it is.
Demanding something to be improved is one thing, but you can play ED for what it is and role play heck out of it. Really do not require that much. It requires to let go a bit of your OCD and obsession with targets and goals.