Quelqu'un and its derivatives

Question

Why are quelqu'un, quelqu'une, quelques-uns, quelques-unes left out? These indefinite pronouns are more natural IMO than certain, certains, certaine, certaines and mean the same thing. In both cases you carve out a subset of individuals w/o further identification. Per robert online certain is a synonym. Here is modern and absolute meaning of QUELQU'UN..........
 1  Une personne totalement indéterminée. « On dirait que quelqu'un joue du piano quelque part » (Alain-Fournier).➙ on. Le besoin de parler àquelqu'un.

Accepted Answer

Hi Max,
I do agree that in the case of 'certains d'entre eux' you could replace certain by 'quelques-uns' but quelqu'un and certains have different meanings, a few of them and some of them, subtle may it be...
Of the indefinite adjectives , 
certains is the opposite of tous,
quelque is the opposite of beaucoup and
plusieurs is more than one.
Quelqu'un/une = someone 
quelques-uns/unes = a few of them
Just one of the beautiful complexity of the French language...

Answer

Your position, that "certains is the opposite of tous," is not supported by le petit robert, which lists the folllowing opposites (none being tous) for certain(s): 
■ CONTRAIRES : 1. Incertain ; contestable, controversé, discutable, douteux, erroné, 1. faux ; aléatoire, improbable ; hésitant, sceptique.© 2018 Dictionnaires Le Robert - Le Petit Robert de la langue française
While I do agree that certains is a subset of tous, I do not agree that certains is an opposite or antonym of tous. I do agree that that is a certain (pun intended) fuzziness in the idea of an opposite - at least in English. I believe that an opposite is something we agree to call an opposite. We may therefore simply disagree on our conception of opposite.
In Robert I found this definition and examples for certains.
 3  PLUR. Quelques-uns parmi d'autres. Certains peuples. Dans certains pays.➙ quelque. En certaines occasions. À certains moments.
© 2018 Dictionnaires Le Robert - Le Petit Robert de la langue française
Again, the only synonyms of quelque listed by Robert are found in its definition.
 1  AU SING. LITTÉR. Un, certain.
© 2018 Dictionnaires Le Robert - Le Petit Robert de la langue française
Certain and quelque appear to be perfectly synonymous and more or less interchangeable. I speak French every day and do not hear or feel a difference between the two. 
Moving on, Robert does not support your contention that "quelque is the opposite of beaucoup". In fact, le petit robert lists no antonyms whatsover for quelque. Moreover, beaucoup does not appear in robert's definition of quelque. Be that as it may, quelque-uns may be a subset of beaucoup in the way certains can be a subset of tous. While I don't see them as opposites, you may. Here again the problem seems to be the fluid English definition of opposite. And, for what it's worth hear, French contraire and English contrary are classic faux amis. Another issue....
I agree that "plusieurs is more than one."  I don't recall taking an incompatible position 0n plusieurs. Nor do I contest that "Quelqu'un/une = someone" and "quelques-uns/unes = a few of them"
I don't agree that quelque versus certains matter is "Just one of the beautiful complexity (sic) of the French language." I think it fact that the French language is quite clear on the matter, at least according to le petit robert: certain and quelque are synomymous. Furthermore, I find no support for your postions in Word Refernence or Collins - references of IMO lesser quality that the revered Robert. If there is an understood or felt distinction between the two words, I would think some lexicographer would be on top of this - especially in words so commonly used.
I would like to hear from our native French speakers on this matter. I will also appreciate your reponse. I enjoy the debat and am happy to hear oppositing opinions. Whatever transpires, we will all know French a little better as a result.