Why do attempted murder and murder carry different sentences?

Attempted murder and murder are two very similar crimes. Often, the only difference between the two is that the victim did not die and is therefore not a murder victim. For example, if a man shoots someone in the chest, but EMT’s arrive before the victim dies from the bullet wound, then it is not murder and therefore the sentence would not be as severe for the shooter. However, if the EMT’s show up thirty minutes later for some reason or other, the victim could have died so it’s murder. The only difference between these two scenarios is the time in which the EMT’s are able to get to the victim.

So, why does attempted murder warrant a different sentence than murder?

It is entirely possible that I asked this question before. I did a quick search before I asked it and couldn’t find anything but in case I have asked it already, I apologize.