Since the trunk open 'button' is on a touchscreen, I can see an additional issue with allowing it to be opened in Drive. It's much easier to accidentally press the wrong part of the screen while trying to use it when compared to a physical button dedicated to opening the trunk. They'd probably have to hide the trunk open button behind a menu to make sure it wasn't accessible on the main screen while driving.
The way it works now: You press the Park button and the display changes to show trunk open button. You press trunk open. With foot on brake, press the stalk down. You're back in Drive before the trunk has even opened, never mind opened, accessed by someone, and closed. Now... if you're the getaway driver for the bank job, and you're approaching the bank when you realize you forgot to open the trunk so your partner can jump in as you roll past at speed... yes, you have a problem.You'll need to stop and switch into park to open the trunk... then wait for it to open enough for a person to jump in. The split second timing your gang practiced is ruined. You're definitely going to jail...