The College Application Process has 3 major parts: GPA, SAT/ACT, and Personal (extracurriculars, honors & achievements, community service).
Since every high school has a different curriculum and difficulty level of the courses it offers, your GPA is recalculated based on your course load. Harder courses weigh more than easier ones do.
As for extracurriculars: who’s to say that 500 hours spent training for Varsity Soccer are better than 500 spent serving food at a soup kitchen? How can you judge that?
So, basically, SATs/ACTs are the only consistent part of everyone’s application. With that said, not all universities require them so it’s important to check a university’s admissions page for its list of application requirements.