Getting into one of the top STEM schools is, for everyone, definitely against the odds. From what I have seen the top schools prefer students from schools that they know their school’s track record. They can judge a applicant’s GPA if they have experience with a prior admit. They also want to see that the applicant took the hardest classes and did well in them.
That said, the top schools will admit applicants from most every high school.
I don’t envy the staff that has to make the admit / deny decisions. Must be so hard.