That's not true. There are many scenarios how one can live legally in the US for more than 6 years as a non-immigrant. E.g F-1 student visa for 4+2+4 years (Bachelors through PhD), followed by 3 years on OPT, followed by 6 years on H-1B. That's 19 years.
In addition, nobody "makes" you get a green card.