You ask about key signatures, but there isn't a key signature anywhere in this question!
You're referring to Roman Numeral Analysis (RNA) and symbols like
I V IV I and the system where upper case denotes major chords.
If you are talking about common practice (a.k.a. classical) music, then symbols like
I for a major tonic, or
i for a minor tonic, will more or less tell you whether the music is in a major or minor key. But that isn't always the case, and you should give the key before the numerals. Something like this demonstrates:
Cm: i I7/iv iv V, notice that both lowercase
i and uppercase
I are given. If you have a good understanding of harmony, you can tell from the numerals alone that it is minor key, but the label
Cm: is what really gives the key.
Anyway, RNA can be applied to rock and pop harmony, but you need to be careful about how to interpret the notion of "key." In a progression like
I ♭III IV I all the chords are major triads, but chord roots come from a minor scale. This mixture of major and minor in rock/pop music is very common. It isn't really pure major or minor key, the name chromatic-minor has been proposed for the tonality.
But, sometimes rock/pop uses progressions that are easily placed into major keys, for example
I vi IV V.
When I look for rock chord progressions on the net, I see major key signatures. Why is that?
Notwithstanding what I said above about major/minor ambiguity in rock/pop harmony you might say major key progressions are more popular, but there certainly are minor chord progressions. Style will matter a lot. It's hard to imagine exploring hard rock or metal and not realizing that lots and lots of that music is minor key or minor mode. You probably aren't selecting good sources for progressions. Two very common minor progressions are
i ♭VII ♭VI V and
♭VI ♭VII i.
But when I tried to Google "rock chord progressions"...
Add the word you're looking for: "minor."
When I search "rock music minor chord progressions" at Google, these links come up on the first page of results:
The videos you posted aren't minor key. But, don't over emphasize the emotional happy/sad dichotomy of major/minor. Tempo, rhythm, dynamics, timbre, etc. hugely important emotional factors. Playing loud and fast, even in a major key, can sound aggressive or angry. Playing slow with soft tones in a major key can create a sad mood. Switching to slow, minor could make sad feel tragic. A lot depends on what exactly "sad" is supposed to mean. The musical factors going into creating an emotion are simply more complex that major=happy & minor=sad. There are more than two emotions.