I understand กิเลศ from a Pali-English Dictionary :

Kilesa (and klesa) [from kilissati] 1. stain, soil, impurity, fig. affliction; in a moral sense, depravity, lust. Its occurrence in the Piṭakas is rare; in later works, very frequent, where it is approx. tantamount to our terms lower, or unregenerate nature, sinful desires, vices passions.

kilesa also occurs in a series explanatory of taṇhā, in the stereotype combn of t., diṭṭhi, kilesa "clinging to existence, false ideas and lust" (see Nd2 s. v. taṇhā v.). -- 4. In the same function it stands with rāga, viz rāga dosa moha kilesa, i. e. sensuality, bewilderment and lust (see Nd2 s. v. rāga ii.)...

As you say there is no explanation of the origin or purpose of 'kilesa' or why people are born with it. It seems that the definition associated with clinging to existence (self preservation), false ideas (self excuses/self image) and lust (desire for self-satisfaction) may be the one you see kilesa is about. Is there a basis for that interpretation?

I highlighted a point in italic and blue that says kilesa is rarely mentioned in "the tipitaka" but expounded in the commentaries and latter works. Does this mean the Buddha did not given enough attention to kilesa but the latter followers did (wonder and try to explain -- as you did)?