They occasionally attempt it but Its very ham fisted. The way they try and humanise them in-game (the ones whose likeness are in the game) just doesn't work - They try and force the in-jokes on people who aren't in on the joke or even familiar with the player, so it's just weird and lame.That seems like the biggest issue to me as well. The GT competitors are just names to most people. PD don't allow them to bring their own personality to the table or create a "brand image", they're all just "Dude or dudette in a red t-shirt number 4".
There is a reason real racing with people in the cars is a billion times more popular than RC racing, people like to root for and follow the humans, they attach onto personalities they like. Cars just racing nicely isn't enough.