That's a fallacy. Of course every place in every piece of fiction is by definition fictional. Of course even real people become fictional when they're used as characters in a novel or a movie. In a way, the Earth of Mass Effect is not anymore "the real Earth" than the Earth of Sense and Sensibility is. But the attachment, the feelings of compassion, those that make it possible to feel any connection to fictional characters, are real. The Earth of Mass Effect is a fictional place, that's true, but it is also Earth, and we feel a connection to it - except our psychopathic friends - because Earth is everything we know.
The difference between "Sense and Sensibility" and "Mass Effect 3" is Sense and Sensibility was never about the Earth. At all. Do movie or game audiences always go into a storyline and say "I care about this because it has an Earth in it"? I know I don't. Heck, Star Wars doesn't even have an Earth, and that's one of the most successful science fiction franchises of all time.
And they way describe the "connection" people have towards Earth is a hollow one at best. Instead of actually implementing Earth into Mass Effect 3's story in a meaningful way, the writers literally fell on the "gamers will care, because they live on Earth too!" trope. When you push everything down to a planet that up until this point hasn't had any significance to the Mass Effect series, you're pretty much fighting for something that's only unique to this game, and I personally feel that was a mistake. We got to know so much of all the other species of the galaxy, all their different cultures and how they're so different from us that the final part in this epic series is to save the planet Earth.
And my final point, which I really think speaks volumes in how a lot of Science Fiction series today don't seem to catch that the first two Mass Effect games did quite well,
EARTH IS NOT WHAT MAKES MASS EFFECT UNIQUE.
Halo has you rescue Earth specifically from an evil alien race on many occasions, Command & Conquer has you save the Earth from a hidden alien menace, Resistance is about humans fighting alien invaders, ect. There are plenty of games out there that have Earth as the central target for opposing alien races, and many of them do a better job at making the players care for Earth much better than Mass Effect does.
The first two Mass Effect games did such a great job at breaking the reliance on Earth storyline and it really helped set it apart from all the other action/scifi games that were out there already. For the third and final installment of the series (Or Shepard series), it disappoints me to no end that it all came down to "Earth is special and in danger!" for Shepard's final story.