I have heard that lord parashurama is living as chiranjeevi in the hills of mahendra then what is a need of another avatars?
why lord parashurama can't destroy adharma throughout the yugas?
It should be understood that Lord Parashurama was an avesha avatara. That means he was designated to be an avatara for a specific purpose for specific period of time.
He technically ceased to be an avatara when he met Sri rama who was already an avatara.
That means his goals were accomplished and therein he continued to live as a rishi and guru.
Dharma and adharma coexist always. Adharma can be controlled to an extent by special avataras or prophets but it cannot be abolished forever. This is the law of nature.