Spite. Many people are no more emotionally advanced than a toddler. They're mad about something (traffic, the looming spectre of mortality, queer people, being out of shape, whatever) so they lash out at whatever. Bike lanes are a symbol of things they don't like, and easier to attack than the real sources of their unhappiness.

Is it wide enough to ge another lane for cars? Would that actually reduce traffic at all?

Im not anti-car, but is there any benefit to this? It seems really unlikely