Why were Hobbits not included in the old lists?

Question

In the chapter on "Treebeard" in The Two Towers, we find that Treebeard, upon first encountering the Hobbits, has no recollection of their kind in the "old lists" the Elves had made regarding the creatures of Middle-Earth.

The reasonable assumption behind this question is that Treebeard was not forgetful, and in fact there was no listing of Hobbits in the "old lists" he learned when young (which, considering he is [one of] the oldest living thing[s] in Middle-earth, is a long time ago). Merry is in fact not surprised, for he notes:

We always seem to have got left out of the old lists, and the old stories.

So Pippin even offers inserting a new line:

Half-grown hobbits, the hole-dwellers.

Put us in amongst the four, next to Man (the Big People) and you've got it

Of course, putting them in that location means the list wording would have to change to "five," not "four" free peoples. But the fact that Treebeard was not forgetful is confirmed when during the Entmoot, no others are able to place them, and in fact they vote to add a new line:

They [the Ents] have agreed that you are not Orcs, and that a new line shall be put in the old lists.

So my question is, why did the Elves leave out the Hobbits when giving the list to the Ents? Are Hobbits a younger creation than all other creatures included on that list? Was there some reason the Elves did not include them at all, but especially under the free people groups? Does Tolkien ever address this further?

Answer 1

Hobbits originated as a subgroup of men, but no one knows exactly when/how/where the original split occurred. The elves did not tell the ents because either it had not happened yet or those elves were not yet aware of it. See this question for more detail:

How were hobbits created?

Answer 2

Because they didn't know about them at the time

Hobbits are generally left out of history because they're a fairly secretive people and, for much of their early history, weren't overly concerned with the affairs of other races¹; see the Prologue, for example:

Hobbits are an unobtrusive but very ancient people, [...] Even in ancient days they were, as a rule, shy of 'the Big Folk', as they call us, and now they avoid us with dismay and are becoming hard to find.[...] They possessed from the first the art of disappearing swiftly and silently, when large folk whom they do not wish to meet come blundering by

The beginning of Hobbits lies far back in the Elder Days that are now lost and forgotten. Only the Elves still preserve any records of that vanished time, and their traditions are concerned almost entirely with their own history, in which Men appear seldom and Hobbits are not mentioned at all. Yet it is clear that Hobbits had, in fact, lived quietly in Middle-earth for many long years before other folk became even aware of them.

^{Fellowship of the Ring Prologue 1: "Concerning Hobbits"}

In fact, hobbits don't appear in any history until well into the Third Age:

1050 The Periannath are first mentioned in records, with the coming of the Harfoots to Eriador.

^{Return of the King Appendix B: "The Tale of Years" (ii) The ThirdAge}

Considering Treebeard's remarks about having learned the Old Lists in his youth, we have to date them to sometime in the First Age, long before hobbits entered the historical record.

They were hanging out in different parts of the world

This is a related point to the above, but is worth expanding on; during the First Age, Elvish interests were primarily focused in Beleriand. Aside from the Silvan elves, there hadn't been a significant Elvish presence in Eriador or Rhovanion since the Years of the Trees, long before Men (and, by extension, Hobbits) even existed.

Hobbits, in contrast, were hanging out east of the Misty Mountains until well into the Third Age:

It is clear, nonetheless, from these legends, and from the evidence of their peculiar words and customs, that like many other folk Hobbits had in the distant past moved westward. Their earliest tales seem to glimpse a time when they dwelt in the upper vales of Anduin, between the eaves of Greenwood the Great and the Misty Mountains.

^{Fellowship of the Ring Prologue 1: "Concerning Hobbits"}

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¹ Though they were always friendlier with the Dwarves; another highly insular and secretive people