Well, but there are, aren’t there? The Miller tells the story to the pilgrims, but who tells it to us?
The Miller’s Tale is indisputably a fabliau, however you choose to define that genre. It contains sex romps, lascivious descriptions of the female body, explicit language, fools being duped, savoir conquering avoir, subversion of the regular social order, and of course farting. As such, it’s exactly the sort of tale the drunken Miller would tell, particularly in the context of “quiting” the Knight’s tale of chivalry and nobility. The trouble is, the tale we actually have is beyond the capabilities of that man "that for dronken was al pale, / So that unnethe [hardly] upon his hors he sat" (I A 3120-21). On the most literal level, he’s far too drunk to consistently rhyme a long poem to that standard. If you protest that ‘twere to consider too curiously to consider thus, do we believe the Miller has the vocabulary to discuss in such detail the workings of alchemy? or the university learning to give us Nicholas’ speeches and quote Solomon? Most seriously, the subtle little moments that warn of distant judgement and remind us of morality and sin could never have come from the Miller, to undermine the characters whose antics he seems to relish so.
And thus lith [lie] Alison and NicholasThe passing of time during their bedroom frolics could be described or marked in any number of ways, but this and the continual glancing references to the social presence of religion (Absolom first sees Alisoun at mass, etc) shape a subtle commentary to the decidedly profane actions in counterpoint to them, well beyond the abilities - or interest - of the Miller.
In bisynesse of myrthe and of solas,
Til that the belle of laudes gan to rynge,
And freres [friars] in the chauncel gonne [began to] synge. (I A 3653-56)
So, let us hypothesise the presence of another author/narrator here, and let us call him... Geoffrey?
The Canterbury Tales is presented as a recounting of pilgrim-Chaucer's journey and the tales the pilgrims tell to each other. But are we meant to be experiencing it as pilgrim-Chaucer does - hearing the words of the others just as he does - or are we supposed to be reading his later retelling of it, reshaped through the poet's pen into the form of verse? One narrator, laid over the other, doubling and reshaping what the first has given us? I'm probably considering too curiously again, but distinguishing between the Miller and pilgrim-Chaucer would imply a division, and possibly contention, between type/content and form/detail of the poem. This double-narrator effect would account for the internal contradictions, and the impression the tale gives of mocking itself.
The same theory could then be extended to The Reeve's Tale, in which the malicious delight of the narrator in tearing down the miller and seeing his wife and daughter raped/seduced is underminded by quiet details of the characterisation of the students, the women, and the narrator himself. The solemn, high tone of the Knight in telling his tale is undercut by the bickering of Palamoun and Arcites, by the initial uncertainty of the narrator, by Theseus' possibly self-serving attitude to ruling, and the Prioress' self-righteous hysteria by the tiresome depiction of little Hugh and the extravagance of the Jews' eviler-than-Dr-Evil evilness.
The same quietly sardonic voice undercuts the apparent tone of all those tales, and it's tempting to ascribe it to Chaucer. It's tempting to use the double-narrator idea to read, dissect, pin down, all the tales and the intermediary scenes. But it doesn't really hold up as a consistent theory.
Firstly, which Chaucer are we talking about - Chaucer-the-pilgrim or Chaucer-the-poet (and is there really any point distinguishing between them)? If the first, can we believe that the wide-eyed, eager pilgrim of the Prologue and the bombastic poet of Sir Thopas could slyly undermine the stories he retells in this way? If the second - well, we knew Chaucer was the poet anyway, so ascribing the retelling to him demolishes the whole point of the double-narrator theory altogether.
Secondly, the same sardonic voice is present in every other poem of Chaucer's that I've read, despite the lack of narrator-layering. In fact, Chaucer's ability to simultaneously construct and demolish is (to my mind) one of the most characteristic things about him.
Thirdly, the theory itself is mostly an extension of an assumption which rests on taking the characters entirely too literally, as real people rather than poetic creations in themselves.
But surely we're supposed to believe in these characters? If not, where's the story? And if so, surely by the same token we are meant to take the Knight seriously, and feel the same lasciviousness regarding Alisoun as the Miller does in describing her, and want to see and enjoy the Friar's Summoner getting his come-uppance, as all summoners we know deserve. We have to be involved on an emotional level with the ostensible intent of the story, not only because it makes the subtle questioning more effective and potent if we feel ourselves morally implicated as well as the characters, but because it simply makes a better story that way.
Quotes from the Canterbury Tales taken, of course, from The Riverside Chaucer, ed. Benson, 3rd ed. Boston: Houghton Mifflin, 1987.