Decisions may only seem “highly random” to applicants because the processes are typically opaque. Even schools that just do admissions by stats often do not publish their thresholds. Even when the schools are more transparent than typical, applicants often fail to notice things like the school having different admission buckets (e.g. by major) so that the overall published admission stats in the common data set may not apply. They may also not realize that different schools emphasize different factors (e.g. UCs are much more GPA-heavy and test-score-light than many applicants assume).
Yes, any time there is a subjectively graded part of the admissions process, there is some element of chance, such as getting an “easy grader” or “hard grader” among the admissions readers, or getting an admissions reader who is particularly impressed by one’s specific activity. But that is probably not as big a factor, at least over applications to many schools, as many assume.