1) The Veyron is Standard because it was imported from GT PSP. All of the game's Standards were ported from a previous title, which is why they're sub-standard. All of the Standard models came from PSP, PS2, or even PSX.
2) The Veyron isn't anything special, so it doesn't deserve upgrade to Premium any more than any other car. It's impressive on paper, but that's where it ends.
3) They don't have cockpits because they're last-gen models, or worse, and they didn't have cockpits in the older games.