All of this Shaytan convinced the people that it was for a good cause: to remember the good-doers! Soon, Shaytan convinced the people to put up pictures of the pious people…then, he convinced them to go to their graves to worship…then, Shaytan told them to ask Allah for things by invoking the names of these saints…eventually, the people started using these saints as Waseelah, or intermediaries to Allah. They would invoke the names of the saints, and they would ask them for special favors. These people–the originators of Shirk–then crossed over to the obvious eventuality: they began worshipping the saints.
The Shia have fallen into the same trap as the people of Prophet Nuh (عليه السلام). They have done the same with their Imams and saints. The Shia have erected shrines for their Imams and saints. They put up pictures of them and they invoke their names in prayer. Some even do Tasbeeh to their names. They ask their dead Imams and saints for special favors and believe in Waseelah through their Imams and saints.
Another example for the Shia to ponder is the example of the Christians, who exalted the status of Prophet Isa (عليه السلام). The Christians say that they love Prophet Isa (عليه السلام) but the truth is that Prophet Isa (عليه السلام) will condemn them on the Day of Judgment. Likewise will Ali (رضّى الله عنه) condemn those Shia who have exalted his status.
Why do people commit Shirk? Shirk is actually a reflection of the base and primitive carnal instincts of barbarian men. Primitive men have poor cognitive abilities and they do not possess the higher mental functions to even think of the Al-Ghaib (the Unseen). They can only see what is in front of their very own eyes. What they cannot see is not a reality, and they cannot deal with that. They are like animals; for example, the deer only reacts to a car that is about to hit it, but it does not possess the cognitive ability to think that maybe it should avoid roads because cars come there. It cannot imagine the cars, but can only see them when it is far too late, and the car comes crashing into it.
Primitive man needs to worship whatever it can see: the sun, the moon, trees, rocks, people, shrines, etc. These are physical things and do not require advanced abstraction. But the reality is that “blessed are those who have not seen and yet believe.” Allah Almighty could easily reveal Himself and thus make everyone believe in Him, but the very purpose of the test is to believe in the Unseen and to reject the baseness of this Dunya (materialistic life).
Primitive men cling to polythiestic practises such as Ziaarat, grave-worshipping, Turbah, pictures of their saints, shrines, etc…but Islam came to destroy polythiesm and paganism, and beckons its believers to let go of primitive practises and to instead worship Allah and Allah Alone. Allah is Unseen and yet we believe in His Omnipotency.