Cram schools exist for a reason. You can get lots of help on the ACT/SATs, heck, they even offer it in China.
Grades and extra circular can be gamed as well, but you have to do a lot more planning ahead of time...like you can't just start 6 months before a test date and maybe try again 6 months after that. You have to start form late elementary school and be consistent up until 12th grade.
All the data shows the exact opposite. Also, it's pretty obvious this is false if you spend 10 seconds thinking about it.
You're forced to give the test yourself. Whereas you can easily hire a tutor to read over and "check" (i.e. rewrite) your school essays/homework.
Rich kids can also easily get internships/research experience while in HS due to family connections.