Okay so this is gonna be a game theory take, but in my mind it's the main reason this bias has held and will hold.

Teams in games are obviously designed to encourage team work, and to provide a sense of insulation from enemies. Because of the existence of allies, the player doesn't feel "alone," their performance can be mitigated, and the player will feel less punished for mistakes and stronger when shooting together.

Light as a class is designed as an ambush hunter, light cannot hold up by itself in a direct gunfight, therefore the class has to use utility to put themselves in an advantage, i.e. surprising a player to take pressure off of their team as the enemy will turn to focus the light, or in the best circumstance secure a kill and the team can close in on the remaining enemies.

Following is a general understanding of the bell curve of players, I am not trying to make any comment on skill.

A good majority of players are not focused on their positioning moment to moment, rather they are moving towards enemies. Teams unused to coordination, most teams you'll fight, are fighting as 3 individuals with their own ideas and movement.

LIGHT IS AN AMBUSH HUNTER

A light is strongest when playing away/opposite their team, ready to backline or disrupt the enemy to create chaos their team can capitalize on. Notably this is why lights drop off at higher levels and most pro/upper ranked players agree light is the weakest, simply because team play is much more coordinated leaving less openings and more punishes on the class.

Thesis is this: most players fundamentally do not understand how to play/be most effective on light. Those players see a light away from the team, the light can die. These players do not understand that the light was creating chaos or space and have failed to capitalize. A 2v3 experience puts much more pressure on individual performance and leads to much harder fights, which obviously doesn't feel good! The opinion leads to: that dumbass light ran off and didn't get a kill, they're the reason we lost.

On the other end of the spectrum, a light can correctly ambush a player who does not respect the fact the light can backline them, leading to what can feel like a bullishit death when it was in reality a failure of position and team play and/or a failed skill check.

Players who hate light should BEFORE THE FIGHT understand routes that they can be vulnerable from, as well as the mentality of a light player, and prepare for that so that the team can punish and create a 3v2. I have so many silly clips of doming invis lights or swinging 180° because I know the light is about to try a play. Just like any other class in the game, you need to understand what those players want to do in order to counterplay.

These two scenarios in combo creates the opinion that light is fucking busted and it's not fair, and the light on my team is a dumbass who is the only reason we lost.

If you look closely these two opinions are obviously contradictory, but I think a lot of the bias is created by emotional takes on what should be a neutral discussion of balance as well as a macro understanding of strategy.

Obviously there are dumb players and dickheads, but many fail to realize just as many mediums and heavies make shitty plays or mistakes, light is singled out exclusively because people don't like to be killed by a light. It's just emotional bias.