If its a human invention in your mind, it can also be discarded as such. Whereas as a concept inherent to the universe it can not. Even a dog understands the concept of fairness and empathy and shame from actions they know are wrong. It isn't a human invention. While you may not have any desire to consider morality as unimportant to you as a personal choice, the framework in which you consider it to exist still allows that choice to be made, whereas I cannot, because in the framework I consider it to exist, it can never be discarded as "an invention of humans". The difference is that I can choose to do something immoral, and it would be immoral, and you could do something against the concept of morality and not consider yourself immoral at all by choosing to ignore it as a "human invention".
While some animals may have instincts of their own for shame and other emotions, they're still separate from human morality and in no way subject to it. We've developed our notions of ethics to such a degree that they've become something completely different from what they must have started out as. And funnily enough, in spite of what you say, people who do believe in absolute morality frequently don't seem to believe the things they do to that conflict with that morality to be really immoral because they invent new reasons for why that should be the case. So in that sense, I don't really believe it's much less vulnerable to being "discarded" as you put it.
I don't really want to get into the whole objective vs subjective argument at length here, I've gotten kind of tired of it and anyway it's a topic that's been discussed to death already by better minds than the pair of us. Still, I'll ask you one thing: if you don't believe moral codes come from humanity itself, why have they varied so wildly over the ages and across cultures?
I believe it's closer to a species survival thing, as we're one of the most social races. A society only works if it's members work with each other.
I think this is true of the core impulse to develop morality that people tend to possess, but to a huge degree the actual moral codes people follow are still social constructs, an even vary from person to person.
This discussion has enough meat in it to become another topic.
It does, yeah. I probably CBA to carry on with it much longer but if you want to, go for it.