From Ven. Dhammanando
For eg, why didn't the information that the dhamma was not explained in detail by the aforementioned Buddhas appear as part of a sutta?
As I stated in post #4, this information appears in the Vinaya Piṭaka, not the Vinaya Atthakathā. The Atthakathā merely adds an explanation as to why certain of the previous Buddhas did not teach the Dhamma in detail. This information doesn't appear in the Vinaya because Sāriputta didn't ask about it, being interested only in the issue of the longevity of the former dispensations.
Moreover, the Atthakathā starts its explanation with the word kira, meaning something like "so it is said that,", "it is reported that...", or "rumour has it that..." The use of kira indicates that the commentator is non-committal about the factualness of what he is reporting; if you like, it's commentarial code for "take it with a pinch of salt" or "we don't insist that this is necessarily correct."
If the arahants of the 1st council had heard this info from the Buddha, shouldn't it be part of a sutta?
Not necessarily. If everything had been put into the Suttas then the task of memorizing and transmitting them would have been a far heavier burden than it was. The effect of this would have been that the Sāsanā would have been entirely in the hands of those brahmin converts who were trained memorizers. But we know from the Buddha's prohibition against preserving the teachings in Sanskrit verse that he didn't wish for them to become the exclusive province of just one faction in the sangha.
Even if one doesn't accept the traditional Theravada claim that places the atthakathās at the time of the First Council, still, one must at least accept that there was some information kicking around at that time that was indispensible for a correct understanding of the Suttas, but which was not itself incorporated into the Sutta Piṭaka. To take a simple example, Dhammapada verses 294 reads:
mātaraṃ pitaraṃ hantvā, rājāno dve ca khattiye.
raṭṭhaṃ sānucaraṃ hantvā, anīgho yāti brāhmaṇo ||
After killing his mother, father, and two kṣatriya kings,
And destroying a country, along with its treasurer,
Ungrieving goes the brāhmaṇa.
To ensure that the reader doesn't make the mistake of thinking that the Buddha's teaching was an antinomian one, most translators of the Dhammapada will add a helpful footnote or comments in brackets, explaining that 'mother' = craving, 'father' = conceit, 'kṣatriya kings' = the two extreme views, 'a country' = twelve āyatanas, 'treasurer' = delight and lust, and 'brāhmaṇa' = arahant. But the verse itself doesn't say this, so how do the translators know ? They get the information from the Dhammapada Atthakathā. If this atthakathā didn't exist then we would have no idea at all about the meaning of 'mother', 'father' etc., for these metaphors are not found anywhere else in the Tipiṭaka. And so even if at the First Council there was nothing remotely like the Dhammapada Atthakathā as we have it now, there at least must have existed some agreed understanding among the theras as to how this Dhammapada verse should be construed, but the said understanding being transmitted separately from the Tipiṭaka itself. This agreed understanding of the ancient theras is what "atthakathā" ("talk on meaning") really means.