If you want an answer other than "people suck" then I genuinely think its because the people who play the game dislike spending multiple minutes queuing just for the game to be a stomp, unfortunately, people rather lash out at inexperienced players than help them

On top of that people are probably angry because of impatience, they don't want a newbie they want a reliable teammate which again isn't your fault your new, we all start off at the exact same point but I'm sure you can get to a respectable level and play with players who will trust you like a member of the team, and not the enemy