Because it's against human nature. It's been tested by time, no one can be pure. Everyone has impurities. Unless you're not human(Which is impossible not to be human) Deities supposedly are flawless an pure.
It's possible to be pure, but you'll almost never see it. Someone like Buddha would be. Energy is just all there is, everything else has been made out of it. You can use it for things you consider good or evil.
There's a bit of misunderstanding here, obviously.
To begin with Enlightenment doesn't make you pure, it makes you enlightened. In some people's opinions, enlightenment actually does the exact opposite. Specifically expressed through the story of Adam and Eve (just one of many examples).
Secondly the Great Work expresses the ultimate enlightenment of man, the union between man and the "Divine Mind". Ultimate divinity or final evolution describes a type of ego, this is not man in the form of man becoming pure, it's man in the form of divinity becoming one with perfection. This is basically describing an afterlife kind of thing.
Ipsissimus is the same kind of thing. This is dealing with a higher self type of figure and the unity between it and yourself. However it does not express purity or even perfection, it simply expresses divinity unless being looked at from the Quabalistic perspective which is actually referring to shedding the imperfections of humanity.
So to say humans in this simple form not being capable of absolute purity is true. Even if somehow you manage spiritual purity which you probably can't without some form of ego-death, your physical body is full of impurities. I'd provide you with statistics on that one, but this is not a science website.
Now there are many people in the world who believe you can keep your humanity in the form it currently exists, and still obtain purity. To be honest though it's a belief of those who need to do a bit more research on what they're talking about.
Personally I think the goal for us here, in the current state we're in, should not be trying to rid yourself of impurities completely but to try and understand them and work with them to simply make yourself a better person.