You seem to be a 'deep thinker' as you claim to know what others think. No, I do not know the answer .Most people do not believe in Messengers.If grave worship in your view, is the reason, then why is our Messenger's tomb intact ? How do you know what you have mentioned about the tombs of Abraham, Sara and Isaac in 'Tombs of the Patriarchs' being in Israel ? Are people worshipping these graves? What is the authority of your conclusion ?