Calculators solve problems that have exactly one correct answer. You cannot plagiarize a calculator. They are easy to incorporate into a math curriculum while ensuring that it stays educationally valuable to the students.

LLM's, the internet, even physical books all tend to deal primarily with subjective matters that can be plagiarized. They're not fundamentally different from each other; the more advanced technologies like search engines or LLM's simply make it easier to find relevant content that can be copied. They actually remove the need for students to think for themselves in a way calculators never did. LLM's just make it so easy to commit plagiarism that the system is starting to break down. Plagiarism was always a problem, but it used to be rare enough that the education system could sort-of tolerate it.