To add some excitement. Sometimes I play Solo because I really want to relax and do my trucking in peace. Win an interdiction versus an NPC is easy, even in a T-9 so it is just relaxing and take a drink.
But even in the "dangerous" open, the attacks "just because" have only happened to me twice, the rest were guys who actually played the "Pirate role" or because I was acting against their BGS goals.
If I'm going to an Engineer or it is a CG I would switch to solo, albeit doing CG in Open is a little inspiring as you see other players doing the same than me for an established goal. The problem, of course, it is that CG is what attracts gankers... so, it is not often.