Discuss and learn French: French vocabulary, French grammar, French culture etc.
Well, if languages didn't have their differences, wouldn't we all be speaking the same language? Why do some languages put the verb last (German) or the verb first (Irish) in sentences? Why in some languages does one need to use the Personal Pronoun (English, French), yet in other languages one doesn't need to use them (Spanish, Italian)? Why is the plural in some languages formed by adding an 's' (English) and in some is formed by adding an 'i' (Italian)? And then there is the whole question of reflexive verbs, which appear in the Romance languages but not in others? I have an Irish teacher who says that "Irish isn't English; Irish isn't French--we have to abide by its rules and no others". To me that is a very good way to approach language. Find out and enjoy why a language is different from others. Embrace and celebrate the differences.
You have a wealth of whys there. I will save that for another day, because my thing with the above examples is really this: Why is it that in order to say the same thing, French and English actually need to treat the definite article differently?
To risk being repetitive, take this most basic expression that surely is shared by all humans:
Being so universal, it's got to have an exact equivalence in English, right? Right, here it is:
See, the definite article is a most potent word; it's presence or absence is not to be taken lightly, much less attributed to happenstance, nor, I must emphasize as a thesis here, brushed off to linguistic customs or grammar. There is something else in the mind that says each of the 2 sentences needs to be just so, to click right, and, more importantly, and weirdly, to express the same idea.
We are looking at 2 things that get to be same by being different.
"That's life." is not an " exact equivalence " for "C'est la vie." (It is the first line of a Sinatra song and I don't think "c'est la vie" would be the only-or even best way of expressing the idea in that case)
If there were any "exact equivalences" between languages then your question would have more force.
Even in english "good day" has different meanings based on the context.
Likewise "how are you?" can just be the opening gambit in a conversation or an enquiry as to the state of the person being addressed.
I am sure your question has validity but I doubt very much that I will ever be able to personally contribute much that is positive to the discussion as it seems to me it very quickly "descends " into the depths of reality vs symbols (in which I have no training) .
Perhaps the answer lies within English itself. Nouns as nouns are neutral in English. Therefore the definite article is neutral. Nouns in French tend to be either masculine or feminine. Therefore, le and la are needed to identify the gender of the noun, which is of importance if agreeing adjectives or adverbs are attached in describing the noun. In English, 'life' is neither masculine nor feminine so to follow your example, one need not have to say "That's the life" but can drop the article all together resulting in "That's life."
And what about those languages that do not have a definite article, as certain Eastern European and Asian languages don't? I had a student from an Eastern European country once write "Director gives actor stage movement." He was writing a sentence in English but using the construction of his own language. The definite article was completely eliminated because it didn't exist in his language.
I am sorry, but I disagree with your last sentence. I look at it as we are looking at the same thing that is expressed in two linguistically different ways.
I think you may be looking for some deep psychological explanation that simply doesn't exist. Broadly speaking, articles/determiners are a structural feature available to languages, and different languages accidentally evolve to use them in different ways.
Note that some of your underlying premises are probably not true. For example, you say that "the definite article is a most potent word". Well, why assume that the definite article is "potent" (and what does that really mean anyway)? Given that practically every noun in French is almost always accompanied by an article, doesn't that if anything make them the opposite of "potent"?
It's also important to stress that labels such as "definite article" are just that: arbitrary made-up labels. In reality, the label is a bit of a misnomer when applied to French: "le", "la" etc don't actually mark 'definiteness' much of the time. (But, so what?)
Incidentally, I should say that it is potentially possible to point to factors that may have influenced the evolution of articles in French. But languages evolve as a complex system, so there will be no single factor responsible, and the factors are going to be essentially grammatical/phonetic in nature rather than anything fluffy to do with French psyche, dress sense, etc.