Warren and Sanders are centrists. Centre-right and centre-left respectively.
Disliking corporate greed is not a left/right or political position, but a moral one. Wouldn't you say the same about other types of greed?
Someone on the left might speak about trade unions, collectivising industry, arming workers, etc.
Is that center right?
Workers, otherwise known as the proletariat, make money from their own labour.
Workers are by definition not capitalists.
You do have far-left organisations in the US, though. There's the CPUSA and the PSL, for example.
Just because electoral politics in a particular country lack one end of the spectrum doesn't mean the spectrum doesn't exist.