Cos a health bar is more real.
It's less "not real" than having a blood splattered pane of glass hanging in front of your eyes. (If you did get blood in your eyes? You'd be blinking them shut from the stinging, and unable to see. You wouldn't get a bunch of obvious bloodsplatter in your vision.)
As much as I hate the whole "oooh, immersion!" stuff, I have to admit that the blood-on-screen is LESS immersive than a health bar.
(Also, we're playing an RPG. We have numbers for things and ratings. We know that we have 150 hitpoints. We don't need a "WW2 Shooter"-like no-health-bar, no-HP damage system. I mean, if we go with the Blood-On-Screen system, we should also go for the full action-game "duck behind cover for 5 seconds to recover all your health" system, right?)