Lost speaks to their curse to be scattered from their lands. Thus the need for an apostle for each tribe to seek them out and minister to them.
They do still exist as tribes in different form perhaps to to interbreeding; but, a family which survives still exists. My name is not Bell, but I'm related to Alex Graham Bell. That family still exists so long as I and my siblings exist among others. What do you do with the ritually pure family of cohan priests in Africa - a secluded community which survives to this day right where history says they should be. Arguing nonexistance because you lost them in the history doesn't mean they no longer exist. It means history lost track of them and didn't pay attention. They themselves have known all along where they were.
I mean that the "tribes" no logner exist -- the people were dispersed among a myriad number of peoples -- mostly middle eaterners, secondly Mediterraneans and Iranis and Indians and lastly Northern Europeans and Orientals and native Americans.