Why wouldn't the game give you experience for killing an enemy?
Sounds right. Your reward is that they normally drop better or rarer items. I don't think I've earned over 10 XP for a kill yet. Even a raider boss I ran into who was wearing Power Armor only gave me 10 XP after I killed him in a tense cat and mouse running firefight. My reward, of course, was chunks of power armor.
Do you a low INT stat ? I get 20 or even 30+ for elite enemies or big creatures.