That's weird, right? I mean, if you need blood for a Klingon oath or a magic ritual, cut yourself (or your friend, or your Sith apprentice) somewhere that's not going to hurt for days and limit the mobility of their hands, somewhere that's easy to bandage to limit the chance of getting infected.
I mean seriously, they always cut the hand instead of scoring, say... the top of the wrist or something. Maybe bloody their nose? That heals pretty quickly. Or if you need a lot of blood, head wounds really gush. Maybe behind the ear?
Just stop, Hollywood.
I think a lot of this started from a few works where the character did it nonchalantly to prove their toughness. Like, "I can't be worried about a little thing like aerocontrols mentioned above, I'm too much of a badass." And of course, now everyone misses the significance and just go with it because it's dramatic. When in fact, slashing your scalp or punching your own nose would make even more of an impression the first time someone did it on-screen.
"Sometimes unhinged, sometimes unfair, always entertaining"
- The Crownless
“Tolerance is the virtue of the man without convictions.” GK Chesteron
Deus Vult!