This assumes mice would gain an evolutionary advantage by being venomous. Mice survive as a species by reproducing fast and hiding, not by biting other animals

This assumes mice would gain an evolutionary advantage by being venomous. Mice survive as a species by reproducing fast and hiding, not by biting other animals to teach them a lesson. In order to exploit venom's potential they would need to undo their risk-averse behavior around other animals, which may not be in their best interest as a species.