Humans are one of the most predominant species, if not the most. They are usually the majority in most areas. In GURPS they are worth 0-points (the assumed default).
There are many different ethnicities and nations among the humans, and if they're listed as sub races it doesn't indicate that they're actual sub races of humans (i.e. with their own Racial Template, although a cultural template is likely)—it's just convenient.