Since there are many aspirated stops and affricatives in the world's languages, why there are few aspirated fricatives in the world? Are there any differences per se between them that make it hard to exist?
Aspiration is usually defined as a distinctive increase in voice onset time between the release of a consonant and the initiation of voicing on the segment after the release of the consonant. This is easy to determine in the case of stops, where there is a period of near-silence and then a burst of air due to the release of pressure behind the complete closure of the stop, plus some lag time that is the "aspiration". Fricatives have no complete closure so they have negligible pressure buildup, thus it is very difficult to find the landmarks that would define "aspiration" in a fricative. In fact, it is variously proposed that most fricatives are redundantly aspirated, so the claim about rareness of aspirated fricatives is only supported as a claim about contrasts, and not about an absolute phonetic property (which fricatives can be argued to have by nature). There are a few languages that do have distinctive aspiration, including Korean where under one analysis the fricative phoneme ㅅ is treated as /s/ and ㅆ is treated as /s*/, the star denoting "tense" or "glottalized, constricted" (sometimes "long"). Recent sound changes have modified the pronunciation of the consonants so that ㅆ is credible treated as "plain", which is still different from ㅅ, which always has been perceptibly "aspirated" sounding. Under that analysis, Korean has aspirated and plain /s/, but there is a competing phonological analysis that it has plain and tense /s/, and perceived aspiration of ㅅ is just a phonetic detail.