Why is the division between east side and west side so big?

I was talking to someone I know who lives on the east side and said they never go to the west side and infact actively avoid it. I also keep seeing discussion on the internet along the lines of “I hope we get [x development] on [east/west] side.” I know the bridges across the river can be nightmarish to navigate sometimes but no more so than some major intra-side freeways. So why is the distinction between the two talked about so much?