I think what you're missing is that, in the real world, sample sizes are limited by time, budget and recruitment. Statistical power grows slowly with sample siz

I think what you're missing is that, in the real world, sample sizes are limited by time, budget and recruitment. Statistical power grows slowly with sample size, so fixing potential sampling errors by increasing sample size is a game of diminishing returns. Anything you can do up front to focus your testing on the variable that you want to measure, the better.