Worship of dead people is not a new form of idolatry. We are used to thinking of ancient idolatry as being directed towards images, however kivrei tzadikim [graves of the righteous] was also an ancient form of worship as we see in the Clemintine Recognitions [Book 10: 25]:
Chapter XXV.-Dead Men Deified.
"But if they choose to argue, and affirm that these are rather the places of their birth than of their burial or death, the former and ancient doings shall be convicted from those at hand and still recent, since we have shown that they worship those whom they themselves confess to have been men, and to have died, or rather to have been punished; as the Syrians worship Adonis, and the Egyptians Osiris; the Trojans, Hector; Achilles is worshiped at Leuconesus, Patroclus at Pontus, Alexander the Macedonian at Rhodes; and many others are worshiped, one in one place and another in another, whom they do not doubt to have been dead men. Whence it follows that their predecessors also, falling into a like error, conferred divine honor upon dead men, who perhaps had had some power or some skill, and especially if they had stupefied stolid men by magical fantasies¹¹