There's resistance towards "creaming" elite students out of classrooms, as it can increase a teacher's workload: all the students who require zero assistance and could possibly assist their peers are now removed from class, and replaced with different students who will require more of the teacher's attention, reducing individual attention on students who need help.
There's also some equity concerns, where there's reluctance to invest additional personnel and administrative overhead on students who are "going to be fine no matter what" and would rather see those additional resources towards students who are struggling - lots of this is based on the belief that low-SES students are less likely to be part of any gifted/honors program compared to high-SES students.
Edit: And finally, there's the cynical data-driven reason that underscores this, where if you need to maximize the number of students who demonstrate reading/math proficiency, you need to pull as many students as possible over the line - once they're over the line you can "ignore" them and focus on getting others over the line. Schools unfortunately aren't measured by how they serve their most dedicated students.