Why do I lose LP when my team decides to int and go afk

So I went in a ranked game like everyother game. The game started and my ally midlaner (kayle) got killed in the jungle at minute 1. He starts complaining in chat and got my ADC tilted. Kayle went to lane, gave 3 kills to the enemy mid laner draven en went afk. We did not win the game bcuz we were not able to kill draven as he had 3 kills that early in the game. So I ask myself why I lose LP and the enemy gained LP bcuz someone on my team decided to dc.
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