I see two intertwined main reasons:
- Vanity: to "enjoy" their overpriced toys otherwise than through the clunky cockpit
- Cockpits are so badly thought, rickety and cluttered it's a PITA to play with
The cockpits are a problem at the moment - visibility is really poor when flying around, and don't get me started on the "landing" mechanics, such as they are... I've heard it said that the cockpit clutter is a way of balancing the ships so no one ship is a game killer, but this current iteration is nonsensical. What's the point of having lovely space views if you can barely see them from the cockpit?
Then again, CR is all about the cinematics - maybe the game is meant to be played in third person?