While I am happy you are going with the simplest approach, which in my opinion for your case would be best, is to ignore any attempt to explain ressurection whatsoever. I do want to post a follow up on your post.
When you started this topic you chose as title "reviving/resurrecting players in a semi-realistic rpg". Because, you wanted a more realistic approach you basically eliminated the ability to ressurect/revive from a deceased state, since that is pretty unrealistic. Now in your follow up post, I have to question just how realistic is your game? I cannot imagine any realistic situation where you could explain the need for someone to intentionally die, unless he/she is suicidal.
Since your game is from what I can tell a shoot 'em up type of game, I really do not think you will have to bother with explaining why the player can try again and again. These type of games are carried by the impact the action has, never their story and/or lore. Though more experienced people with this genre can probably give examples that refute my point. What you need to focus on is thus the impact of the game, ensuring the killing is fun and you can do it in a numerous fun and creative ways, because if you make that part boring your game will fail.
I have been told Killing Floor is a great example of the genre, perhaps check it out, though I think you already have.
Conclusion for this is to focus on the killing and less on the lore, because in shoot 'em ups the action/flow is what is important.