See this video (a captioned children's story) at 1:34 (1 minute & 34 seconds)
They say, "He becomes even more richer than ever before"
Is more richer idiomatic?
Why don't they say "even much richer" or "even far richer"?
The video seems to me to be poorly made, and I found several more strange errors over the course of a minute or two, so I think "more richer" is just a mistake. For example, here is the first sentence of the video, with errors marked:
Once there was a rich merchant,  he decided to load all his wealth into his two ships to  the  other lands to make more wealth, but alas they  drowned, leaving him poor,  all he was left with was his little son and his  only land left.
 Comma splice (run-on sentence).
 Misuse of "to"; we don't load "to" somewhere.
 "The" other lands, but no other lands have previously been introduced.
 Ships don't "drown", they "sink".
 Comma splice (run-on sentence).
 Should be "only his land left". Also, repeating "left" is awkward.
All except  and  are errors that native speakers do not make.
A slightly ungrammatical or nonstandard phrase like "more richer" can be a playful abuse of grammar for rhetorical effect ("catachresis"), but I don't think that's what's happening in this case.
I think learning English from fairy tales and nursery rhymes is a very good idea. But they need to be in real English, and preferably children's stories or rhymes that most speakers (in at least one country) are familiar with.
In the cited context, even is an optional "intensifier" (in He got [even] richer, the implication is he was already quite rich before the later increase in wealth). But whether or not even is included doesn't affect the grammatical issue as regards how English expresses comparatives and superlatives.
Briefly, monosyllabic adjectives usually take the suffixes -er, est, whereas [most] disyllabic adjectives (apart from those ending in -y), express the same thing using more, most. But you have to choose one or the other (richer or more rich, not more richer).
OP's "double superlative" example is a relatively uncommon "mistake" that may pass unnoticed by some, while driving others to distraction. Personally don't really think it was ever common in any specific regional or socio-economic dialectal communities. It's just that since all (adult) native speakers should instantly recognize the "error", it's a convenient shorthand way for writers to imply speaker is uneducated.
In practice the usage more richer is semantically equivalent to richerer (both repeat the "comparativizing" format). That specific quirky morphological form isn't so common with rich, but it does come into play more often with irregularly-formed comparatives such as good, better, best.
Hence you'll find many written instances of even triple superlatives, such as more betterer. But such usages would almost always be facetious (speaker is just "playing with language"). Effectively, deliberately applying the kind of "syntactic logic" that a child might make when first discovering the two different ways of forming superlatives, before realizing that they're mutually incompatible (and that -er, -est can't be "intensified" by repetition as in the "childish" usage You're my bestest friend).
But OP's Why don't they say "even much richer" or "even far richer", involves a different issue. They're both syntactically invalid for most contexts, because even, much and far are mutually incompatible "intensifiers" for whatever "comparative adjective" follows. Valid permutations are...
He becomes rich (by implication, he wasn't rich before)
He becomes more rich (by implication, he was already somewhat rich)
He becomes richer (that "already rich" implication applies to the following "emphatic" versions)
He becomes much more rich
He becomes far more rich
He becomes even more rich
He becomes much richer
He becomes far richer
He becomes even richer