For an international student, graduating with a biology degree would mean you have almost no chance in finding a job – there are too many American biology majors, so there is no incentive for an employer to sponsor you for a work visa. Most international students either go to graduate school or marry a citizen in order to stay after their OPT.
Master’s programs are expensive – they cost as much/more than undergraduate and there’s little/no financial aid for international students. But most good PhD programs are funded, meaning the students don’t pay tuition, and receive a stipend to live on.