The teacher chooses the exact mix per class, so it's possible you fail even if you ace the exams, or pass even if you fail the exams.
This variability is why the standardized tests exist.
I guess it makes sense to do as little as possible to scrape a C or whatever the passing grade is while at high-school and spend all your time optimising for the SAT then instead? Or not that simple?
College admissions are similarly a blend of different criteria: race, high school grades, did one of your parents go to that school (legacy), did your family donate to the school, are you a good athlete, are you a good musician, and of course what is your ACT or SAT score.
The exact weightings are not known to the students applying, but it's safe to say you can't just minmax on the standardized test.
I don't know if you've picked up the tone of some of the other comments in this thread, but people feel very strongly about what factors should be considered in college admissions. Almost no people will defend legacy admissions, few will defend athletic admissions, some will defend musicians, many will defend GPA and standardized testing.
College admissions are zero sum, and different weightings benefit one group over another. For example, "Asians" (the US does not differentiate South Asian [India, etc] from East Asians [China, Korea, Japan]) have very high test scores in the US. If you base your college admissions just on taking the people with the highest test scores, you would get a massive over-representation of Asians. Schools have decided that isn't what they want, so they take people with worse test scores in order to "balance" the student population.
This infuriates the people passed over in the name of "balance", naturally.
* Graded homework
* Papers/Project/Labs
* Midterm/Final Exam
The teacher chooses the exact mix per class, so it's possible you fail even if you ace the exams, or pass even if you fail the exams. This variability is why the standardized tests exist.