-You have fewer students. This is a _much_ bigger deal than you make it out to be, especially at higher grade levels. When you have to give feedback to close to 200 students (over 200 it not unheard of), you can't make it as meaningful as you'd like. There are things you can do with a couple dozen students that simply does not scale.
-You are dealing with an easier subset of children. If their parents want them to have extra lessons, and are both willing and able to pay for it (or even fill out scholarship forms), then you're not dealing with any of the difficult kids. Parents who don't care about school beget kids who don't care, and they are orders of magnitude more difficult to work with.
-Its clear that you are not aware of how much material these standardized tests expect you to cover in a course. I'm sure it varies by state, but this is from the perspective of VA testing, which is where I grew up and where I know many teachers. For just about any tested middle or high school class, they have the material mapped to within 3 or 4 days of the length of the school year. A couple of unexpected days off and they have to start cutting material that the test expects to be covered. There's absolutely no time to do the things you're talking about unless you want to blow off the tests - and then you're out of a job.
Standardized testing, _in its current form_, is an unmitigated disaster for students' educations. If you can say otherwise having actually faced the system you are so happy with (or even talked to a few who have), then we have a conversation.