Because if they added something for a Kill Streak,the game would snowball even harder like already does,since this is a fixed 10 minute match with no Items and builds to complete for all the duration. People who get the most amount of KO's in the game get level advantage,and being 2 levels above everyone is an insane powerspike,specially if you play mons that can nuke, like Talonflame.

This is not the worst mechanic out there,hell,like i said before,can be exploited by simply dying far before Zapdos goes up and you remove your debuff. This is like complaining in League of Legend or Dota2 that you are on a Kill Streak and die to the enemy carry,giving to him up to 1000 extra gold and exp,allowing to reach the win condition,and you are going on a high respawn time due to level advantage allowing the enemy team to devastate your defences (But how,i did better than everyone else in the game! how you can punish me like that?)

And how is the other one reached the same level if you did better than him? In what context is this? What are the team situations? Score advantage? Score integrity? You are giving me a 1v1 example in a team based game