Because of stupid laws in Japan, realistic depiction of sexual organs and pubic hair, used to be banned. The limits of what can be shown is now somewhat ambiguous. Sometimes, just the point of insertion is censored, or the bars are just there for the sake of saying they are there.
I think somehow, the censorship evolved into a fetish on its own for some readers (ie, if it's not censored, some people wouldn't like it as much, or if you see hair, it's gross).
At this point, I think it's just a tradition and to be on the safe side. Very few lawsuits have been made about this.