I thought the same thing too, til I started watching real porn of girls with bad dragon dildos, then I finally realize why people like watching girls get fucked with big dicks.
1. Adds to the domination aspect. For some guys, having the biggest dick is like being the buffest guy at the gym. For others, they like when the girl can't handle the dick and they force it anyways. Same feeling you get from rape content, except it's super vanilla (Stop, stop, it hurts! Fuck you, I'm going all the way in!).
2. More girth "seems" more pleasurable to the girl, also makes any pussy a tighter fit. Makes it more representative of the death grip you got on your dick when jerking lol. It just makes "sense" that if your thick dick is rubbing against more flesh inside the vagina, she's probably feeling a lot more from sex.
Of course, most real girls don't actually like big dicks past a certain point, so it's all done to please the boys.