How quickly we forget. This is not the first time workers have faced inhumane working conditions. Unions worked as a solution before - why wouldn't they work again?
Other would like, or even try. But Amazon management, an enormous American corporations, are determined to not allow to do so. Amazon warehouse workers are, generally, not a particularly wealthy or powerful segment of the society.
But I don't think mandatory unions are a solution. Unions would have to be very popular for such legislation to pass. But if they were so popular, they would no longer need compulsory membership.
In general it is unions and organized labor that push legislation meant to help workers, so starting with legislation instead is putting the cart before the horse.