One of the biggest reasons for using the ACT and the SAT, are because they are the "GREAT EQUALIZER". How many times will you hear on this forum, an applicant claiming/bragging/etc. that they are in a "COMPETITIVE SCHOOL". Well that's all fine and all, but it doesn't say much. This is why the academy requests a "SCHOOL PROFILE". They want to see what classes are available; what classes you are taking; and more importantly, what colleges the average kid in your high school is attending.
Now, with some schools having the complete IB Program; and some schools have the complete AP course load; and some only having Honor's Classes; and some having ALL OF THE ABOVE; and MORE IMPORTANTLY, some having NONE OF THE ABOVE....... How do you compare the 4.0 student at one school; with the 4.0 student from another school. The academy, does all schools, have a very difficult time trying to compare apples with transmissions. But the ACT and SAT are the great equalizer. The kid at the private school in Washington State and the kid at the Public School in Denver and the kid at the charter school in Dallas and the home schooled kid in Miami are ALL TAKING THE EXACT SAME ACT and SAT TESTS. This is one of the main ways of comparing students with different schools and classes.
You are correct, it isn't always the best indicator of a student's ability to learn or succeed academically. However, it is a good indicator. As mentioned, what does it say when a kid has a 3.0 gpa, yet, they have a 33 ACT or 2100 SAT? Or the kid with a 16 ACT and a 4.0 gpa? Now, considering there are so many applicants to the academies, it's convenient when you can see a 3.9-4.0gpa AND a 33+ACT. That tells you that this applicant probably has no issues academically. But this isn't enough for the academy. They don't want just the Brainiacs. There are a lot of 4.0 gpa and 36ACT applicants who have been turned down from the academy. Just like there are plenty of applicants who played 3 sports each year, were captain of the team, had a lot of leadership experience and community service; who had a 2.5gpa and they too were turned down for an appointment. The academy wants BOTH!!! They want the applicant who will succeed academically as well as socially. Which includes leadership, teamwork, community service, sports, etc.
The ACT and SAT are needed. They are the only way to take those applicants from private schools, public schools, charter schools, home schools, etc. and compare them on the same level. The ACT and SAT are the only thing academically that they ALL have in common. They all took the exact same tests. But that's why the academy weights the applications. They know the difference betweena 4.0 in the mandatory state required math class; and the 4.0 gpa in the IB Program Math class. That's why it's so important that you show that you did the BEST YOU COULD, with what was AVAILABLE to you. You won't be marked down for not being in the IB program or taking AP classes if they weren't available. But you WILL be marked down if they were available and you didn't take them.