What car is it? A lot of the cars in GT have their interiors listed as "simple" as an interior wasn't made for the car, so you'll just end up getting a generic, blacked out version.
Those are standard cars with simple interiors. If you'd have bought the evo IX or X you'd have had a premium car with a detailed interior. There are more than 300 premium cars in the game the rest are standard cars.
As @GT needs F1 says above, you have purchased what were known in GT5 as "standard cars", meaning they were ported over from the PS2 era and have no proper interior view, just a black shadow outline of a dash that is the same in all standard cars. You cannot fix this, it's just how the game is designed. How you identify these cars in the dealership is that the premium cars are identified as having "detailed interiors". If you want a full dash, only buy cars listed as having that detailed interior.