[Jianghong94]

I guess the primary reason is that the answers must be numbers that can be verified easily. Otherwise, you just flood the validator with long LLM reasoning that's hard to verify. People have been proposing using LEAN as a medium for answers but AFAIK even LEAN is not mainstream in the general math community, so there's always trade-offs.

Also, coming up with good problems is an art in its own right; the Soviets was famous for institutionalizing anti-Semitism via special math puzzles for Jews in Moscow Univerisity entrance exams. The questions are constructed as such that are hard to solve but have some elementary solutions to divert criticism.