It honestly probably has more to do with how we’re socialized into music. The trope of minor being used for sad things and major being used for happy things is sort of an artificially created thing to an extent.
In fact, not even all very young school children will agree about this until we tell them so (my wife used to teach elementary music). Other cultures with less overbearing exposure to Western music are even less likely to always correctly identify these.
To get more complicated, if you look into Arabic maqams (essentially scales for the sake of simplicity) they sound quite different with a bit more microtonality compared to Western ideas of tuning and such. The thing is, when we hear them, they all just sound semi-foreign. The ones that sound most like a major scale or maybe a lydian scale sound happier to us. The ones closer to a minor or locrian scale sound sadder.
But in the cultures to which these are native, the great variety of maqams hold much more subtley of meaning and even the ones we might equate with sadness are not thought of as such where they are actually commonly used.
Below we go into ELI have a basic understanding of theory:
So major and minor are very black and white for us, but if you just wanna talk about chords, what about bigger chords? Start with something like a CMaj7. It’s a C on the bottom with an Em on the top. Does it sound happy or sad? Are you hearing the C major of the primary triad, or the minor triad when ignoring the root? You can obviously grow this idea out like crazy. A Cm9… what do you hear? The EbMaj7 ignoring the root? The Eb major chord in the middle ignoring root and 9? The Gm on top? The Cm on bottom?
Is this a happy or sad sounding chord? I think most people would call it “dreamy, sleepy, or ethereal,” but once again, that’s likely more to do with how we most often hear it used.
Also, context counts. You can revoice a complicated chord for a different sound. But you can also revoice a simple chord and depending on the context and character of the piece, it might throw you off. Have you ever been listening to something and couldn’t quite tell if a given chord was major or minor in context on the fly? What if you have a C major triad and suddenly the bass moves away from the C up to E and the E and G are sustained in the upper voice. Is this implying an Em without the 5th? Is it still implying the C major but in root position? You probably need context to tell you.
How about a C7. This one is great for context. Do you hear the Edim or the C major? Or hell, do you just hear it as a dominant chord because it’s followed by an F or even Dm?
TL;DR – Because someone told you so.