Why Humans ?
The nature is evolving life from a simple form to a complex one. Every living thing has a role to play in teh universe and for that it has been given a kind of free will to some extent. E g a bee can choose whichever flower it likes. However the level of free will is so low that it does not warrant any accountability. It’s only the human that has evolved so much advanced that the free will he has and the role he has on Earth as a “designer” or “innovator”, carry some risks of misuse. This is what the angels were worried about when God created Adam by giving him the free will and made him a ruler of the resources of Earth. God is managing those risks and governing our free will thru the accountability process.