I think it's just the way it was programmed. We don't have the party function here, and if you aren't partied, by default, you get no XP even if something level 40 dies right next to your level 2, if someone else landed the killing blow.
LoS XP encourages parties to stay together, engage combat together, and remain in battle until it is finished if they are to gain experience. It's not perfect, but I don't see devs modifying the system so that wizards can even more easily drop AOEs on the ground and just walk away as enemies stand still without any reaction to their health dropping. I also understand you want to play the way you like to, just like everyone else, but you don't actually need to walk out of line of sight to execute your strategy with full efficiency.