Why do people constantly leave matches when they start losing

I could be up 10-9 then people leave in round 2 I don’t understand why people do this, do they just get too tired to keep going or are they just so butthurt that they leave after losing even the smallest amount. I really think most of my wins come by people leaving. I know it’s free wins but I wanna work my cardio and combos but as soon as they get rocked they leave.

Do you lose less elo or something from leaving?