Why people consider shia as muslims?

They are polytheists,they worship imam Ali,and when they are in pain or having trouble they say O Ali instead of O Allah! and they claim that Gabriel was meant to give the message to Ali but he lost his way and gave it to muhammad! and when they quote they dont say prophet mohammad said:etc but they say Ali said :etc! so they have their own religion but it does include our quran and they lost after prophet mohammad died just like chrisrians did when prophet jesus left,why people consider them muslims?!