They aren’t, though, unless you assume that a student’s chance at a PhD is completely or primarily a result of the undergraduate school they attend. That simple isn’t true in most (any?) fields. A better measure might be the percentage of students entering a PhD program who were also interested in entering a PhD program.
This is going to vary depending on the field, but at least STEM fields, I have to repeat this quote for emphasis. When I am looking for PhD students, I care a hell of a lot more about whether they have research experience than I do about what school they attended. Whether they got that experience at a big public or a small private or somewhere in between doesn’t so much matter.