Because if morals are subjective their whole way of thinking about them falls apart?
I'm not sure if morals are objective or subjective. If they're subjective, well, then they're still objective in the sense that we're all living things and there are some points of agreement that are for all intensive purposes universal. If they are objective, good luck figuring out what the perfect morally objective path is. There have been many cases where I wished there was a way to do right, but all paths seemed wrong. So I don't know. I wish I did.
The only objective morals I've ever heard of, are either based in religion or in metaphysical "substances" like Plato's Ideas. So the only way to have objective morality would be if there was something like the perfect Good existing somewhere, in the ideal plane. That, or believing that there's an all-knowing god, who tells us what's good and what's bad. For people who believe that, knowing that other cultures have other morals makes no difference, because "good" and "evil" exist somewhere; they're not dependant of people's opinions or culture, just like physics. In fact, it could be possible that nobody agreed with these morals, and nonetheless, they'd still exist and they'd be true.
But there's a fair amount of people who don't believe in god or metaphysics, but that still believe that human rights and the like are good in themselves. I guess they've heard so many times that those rights are good, that they have forgotten that it was a group of persons the ones who established them. These are the ones who make no sense at all: either you believe in God or the "Idea of Good", or you can't have objective morality.
It's much simpler to think things are black and white rather than shades of grey. Generally religious people look for the simplest answers that are easiest to comprehend, require the least thought and understanding. They also generally like closure meaning a full explanation is prefered over a partial one even if the full explanation is completely flawed and the partial one has no flaws. Having an answer is more important than that answer being correct.
The laughable thing is they will claim objective morality and then will break their own rules all the time.
Subjective: Based on one's own perspective.* Objective: Based on another perspective.*
*In Tomist philosophy, the terminology is reversed since the object's action (impyling God is the ultimate Object) is applied on the passive subject (implying Creation is the ultimate Subject).
Absolute: Self-sufficient, free and ultimately axiomatic; a basis of further relation.
Relative: Defined only by its difference when compared to another element. A system of relative elements that does not trace back to an absolute source, as in case of a hermetic circle-relation, can only be nothingness, as it isn't given the principle to take the shape of.
Morals: Rules or laws which determine whether something is right or wrong for the subject to commit (including abstinence).
Determination: Assigning a quality to a subject, thus contrasting it from its environment by differentiation, giving it a unique identity.
If morals are absolute: Morals determine everything (e.g. God).
If morals are relative: Morals do not determine everything, however, they inherit a conditional definitive power (determination of right and wrong).
If morals are objective: Morals come from an environmental source.
If morals are subjective: Morals come from oneself.
We have now defined four potential moralities that may or may not coexist: absolute objective, absolute subjective, relative objective and relative subjective. Further analysis and definition of terms is necessary.
Don't define them separately. Moral objectivism/universalism: "Moral universalism (also called moral objectivism or universal morality) is the meta-ethical position that some system of ethics, or a universal ethic, applies universally, that is, for "all similarly situated individuals", regardless of culture, race, sex, religion, nationality, sexuality, or any other distinguishing feature."
I'm of the opinion that if there is an objective morality, there's two possible forms of it. 1) Genetically programmed. There are behaviors that are programmed as "right" and "wrong" in your temporal and frontal lobes. It's programmed to protect the social structure of social animals. I imagine it's an "objective moral" for ants to defend the nest. You're basically protecting your gene pool by engaging in "morality." 2) An objective, divine Law that pertains to Good and Evil. This is, by virtue of our humanity, only dimly perceived. The holy books of various religions are therefore important to delineate an otherwise limited perspective of "Right" and "Wrong."
However, I'm of the opinion that morality can be both objective AND subjective. This goes back to #1 up there. Some behaviors are coded, and some behaviors are imprinted by experience. You learn socialized behaviors and gain an understanding of what your culture has taught you as "right" and "wrong."
As for #2, who knows? Maybe there IS some divine moral code. We obviously don't understand it, much less follow it. It certainly goes against the very nature of being human.