I have to admit, I don't know. I've done some research and I am not entirely sure. So, I offer two explanations - in two different answers.
The principle of (formulaic) economy
As Russo reminds us,
"Anyone who reads Homer in Greek becomes eventually aware that repetition is constantly at play, some of its forms being more immediately evident than others." (Russo 1997/2011: 252)
Originally, the formula was understood as "a group of words which is regularly employed under the same metrical conditions to express a given essential idea" (Parry 1928, 1930), and that formula should be "made up of at least four words or five syllables, with the exception of noun-epithets, which may be shorter" (Parry 1971: 272, 275 n1, as cited in Russo 1997/2011). Here's a screenshot of a relevant section from Finkelberg 2012:
Later the definition of the formula was revised to "a repeated word group" where "the use of one word created a strong presumption that the other would follow" (Hainsworth 1968, as cited in Edwards 1997/2011: 265; emphasis mine - Alex B.).
The current consensus seems to be that around sixty-seventy percent of Homeric diction is formulaic. Finkelberg 2012 adds that the formulaic and non-formulaic elements were mutually complementary.
So, what is this principle of economy proposed by Milman Parry?
As Steve Reece (Reece 2013) writes,
"By economy Parry meant that generally only one epithet for a character or object was available to fill each common metrical space," [emphasis mine] thus
"Given a particular metrical space to fill, the poet was not required to create a new epithet ex nihilo; he did not even have to pause to consider a choice between two or more inherited epithets – only one epithet was available for that particular metrical space."
Formulaic language in Sappho
"The same forces which created the poetic epic language of Homer created the poetic lyric language of Sappho and Alcaeus. […] Yet while we may feel some doubt as to the way in which they made their verses, there is not the least doubt that their poetic language was drawn from an oral tradition: only in an oral poetry does one ever find such a variety of forms that have each one its own metrical value." (Parry 1932: 29-30)
"The majority of Homeric and later poetic repetitions (actually the whole corpus of hexametric poetry (Epic Meter) in Greek, and still further in the lyric and choral poetry) can be defined as more or less formulaic." (Létoublon 2013)
"From beginning to end, then, Sappho 1 is a work wholly indebted to oral traditional
poetic techniques in terms of its phraseological thematic structuring, its rhetoric, and even its
extralexical encoding of formulaic phraseology, and it was the combination of Sappho's
individual poetic talents with these traditional possibilities that imparted such a powerful impact
to her verses." (Garner :334)
"I offer proof that a composition like Song 44 of Sappho was created by way of a formulaic language that is cognate with the formulaic language used in the compositions that we know as the Homeric Iliad and Odyssey." (Nagy 2015, October 22)