A game like Forza looks extremely appealing on the surface but it doesn't have that much depth, a perfect game for today's fast food YouTube generation with their instant gratification needs. A game like PCARS 2 has a lot more depth but you don't get to experience that from playing the game for a few hours. So all those reviewers that play both games for a few hours will naturally give Forza a higher rating. If they would play both games for a couple of weeks/months I believe things might have a different outcome (depending on how realistic they want it to be).
I see Forza as a great stepping stone for people coming from other games, it's an excellent introduction into the racing genre with all the handholding features like a rewind button, a friction assist and today's addictive internet features like loot crate gambling and levelling up. Some people will be pleased with that and never want anything different and that's fine. But other people - the true Motorsport fans - will find the shortcomings in Forza quite fast and they will want something more realistic. For that audience PCARS 2 is by far the superior game.
In my books FM7 and PCARS2 aren't even competition because the games are so completely different. I preordered them both since I am both a sucker for shiney stuff, levelling, collecting cars loot crates and all that, but also for realism and true Motorsport experiences. But for many people the choise seems to be PCARS 2 or FM7 which I don't really understand since the games are so different.
It's actually great that new-to-racing fans get into FM7 as their first racing game (if the alternative is non-racing games). Those are the potential new customers for PCARS 3 in 2-3 years.