As always you're asking a rhetorical question. I think this dates back to a time, when people which were considered educated could count till four. The limit of a count expanded afterwards till seven, ten, fourty and so on.

because Russian language is complex as it reflects the complicity of the Russian brain... just like in nature, there are worms and there are giraffes... worms are simple and giraffes are complex

"Why is the use of numerals complex in Russian?" No, I don't think it is. Numbers five and up are literally counted as "(Five) of ____". Some of my Slovak students would back-translate that way (same system in most?/all? Slavic languages, I guess). That's easier than counting "scores" in French, and the weird system in this cartoon: http://satwcomic.com/just-a-number

You should understand that the Russians as native speakers use a lot of language stuff unconsciously. When I was in Germany as an exchange student I was asked the same question and I couldn't understand it because I didn't know that is so baffled - the Germans explained it to me that we use 1 -> nominative singular; 2, 3, 4 -> genitive singular; 5 - 20 -> genitive plural, but attention, please! -> 21, 31, 41, ..., 91, 101, 201, ..., 901, 1001, xxx(x)1 -> nominative singular!; 22, 23, 24;32, 33, 34; ...; 92, 93, 94; 102, 103, 104; ..., 902, 903, 904; 1002, 1003 , 1004; ....; xxx(x)2, xxx(x)3, xxx(x)4 -> genitive singular! 5 - 20, 25 - 30, ..., 95 - 100, 105 - 120, 125 - 130, ... -> genitive plural!

Dual is a grammatical number that some languages use in addition to singular and plural. When a noun or pronoun appears in dual form, it is interpreted as referring to precisely two of the entities (objects or persons) identified by the noun or pronoun. Verbs can also have dual agreement forms in these languages. The dual number existed in Proto-Indo-European, persisted in many of the now extinct ancient Indo-European languages that descended from it— Ancient Greek and Gothic for example—and can still be found in a few modern Indo-European languages such as Scottish Gaelic, Slovenian, and Sorbian. Among surviving, ancient languages, Sanskrit uses dual forms across nouns, verbs, and adjectives. Many more modern Indo-European languages show residual traces of the dual, as in the English distinctions both vs. all, either vs. any, neither vs. none, and so on.