Cause they are very similiar. Just like Ringu and Ju-On were very similiar.
They're both ghost stories, of course they're somewhat similar.
Ring/Ringu was about a demonic child, whose ghost reaps it's revenge through a cursed video tape. Ju-on/The Grudge is about two poltergeists who haunt a house and massacre anyone who comes into contact with said house.
I'm not talking about visuals, here, either. The look and tone of the original films are miles apart from eachother.