Engineering itself can “weed out” less motivated students due to curricular rigor (at any school). Some schools have another admission process to the engineering majors after one enrolls (at Purdue, first year pre-engineering frosh need to get a 3.2 GPA to be assured admission to most engineering majors; lower GPA students may be admitted if there is space available in the major).
Also, public schools generally tend to favor the mission of giving more students a chance to prove themselves in college (although not all will succeed), rather than saying that an imperfect high school record makes a student unworthy.
Actually, as one gets away from the super-selective admission schools, academic qualifications (courses, grades, rank, test scores) tend to become more important, since they provide more differentiation among applicants than at super-selective admission schools flooded with 4.0 / 2250+ / 34+ applicants.