I think test prep allowing people to increase SAT scores is actually a useful feature not a bug.
If you’re the kind of person who’s going to put extra effort to add a few points to your results, you are also the kind of person more likely to do well in collage classes.
I agree in general with what your idea, but this sort of assumes a few things that aren’t always true:
1. Examinee is aware of test prep and its potential benefits. Definitely not always true in low socioeconomic status (SES) families, especially ones with no family members who have been to college.
2. Examinee can afford said test prep courses and/or materials. Library and online are “free” options, but we are back to assuming that the examinee has easy access to a library and/or the internet and knows how to find said materials.
There are many, many students who would do well in college if they had some insight on how to do college. The US has a lot of wasted potential due to our public school focus on bringing up the low achievers to the exclusion of developing those with high achiever potential.
Some private schools and most public schools in “good” neighborhoods have programs and cultures that cater to those with high potential, but these schools only address a relatively small portion of the student population with high potential, imho.
It's not really being the "kind of person who’s going to put extra effort" and more the "kind of person who is privileged enough to have parents that care about their education, know that outside resources exist, and have the time and money to utilize those resources".
If you’re the kind of person who’s going to put extra effort to add a few points to your results, you are also the kind of person more likely to do well in collage classes.