Cost.
It's much cheaper for automakers to not stray too far from the parent company's model.
A little insight about Honda/Acura..
About 10 years ago Honda finally decided to go Tier 1 with Acura, they put money and development into Front engine rear drive V8 vehicles, as well as the well known V10 NSX replacement.. the new architecture was pretty much done, running prototypes and all, with the NSX it had completed the end of it's development cycle..
Then 2008 financial crisis hit, Honda put a stop to it all in an instant. Instead they thought they saw the market going south and were in crisis/survival mode (in retrospect an obvious MASSIVE mistake). So they used existing Honda models to develop Acura's current batch of cars, basically you're getting gussied up Accords and Civics.. It's why their Japan designed RLX only sold 260 cars last month, who wants to pay for a 70k Accord, nope everybody rather have a benz/bimmer/lexus and that's not to mention kudos to Hyundai and Cadilac for upping their game..
While the Acura's are great cars in their own right, just not very relevant..
Not to mention lots of internal development/engineering issues they are having which I won't go into..
But light "could" be at the end of the tunnel, I think they are currently considering going Tier 1 again but we won't see those products for years...
A good sign though is the NSX replacement due in 2015, it was supposed to be based off the RLX hybrid drivetrain but turned around, but they found out very early it would never be a F458 competitor, so they engineered all new powertrain, 90degree V6TT which we could see in the next RLX
Anyway in 3/4 years we will see..