Because they would actually have to balance it and that scares them shitless.
>Put it in the same category as LMGs and the like.
>Make it a viable threat to everyone including the user and his team (Enemy pops the fuel tank and he's close to others on his team, they risk catching fire.
>Shit like flame retardant garments reducing risk but not eliminate it.
>Make it heavy enough to burden the user in certain ways like a fuck huge and heavy metal tank on your back would. Banging on doorways or being slower standing up from prone as examples.
>RISK OF SUFFOCATION WHEN USED INDOORS! I can't stress this enough. Not that hard to do, map out the insides as a trigger and add a blurring effect to the player's screen.
>Fire needs to spread when appropriate. IE, learn fire behavior.
>I would also appreciate characters stop, dropping, and rolling to put the fire out.