A likely reason for removing the metaphorical use of "rock" would be the emphasis on monolatry and avoidance of the use of idols which defined the period during which the LXX was produced. The Babylonian exile caused the land to be purged of idols, and the LXX translators "purged" metaphorical uses which might be construed as approving of or using rocks as idols.
The two main Hebrew words meaning rock are צוּר and סלע. It is reasonable to eliminate deficiencies of the translator's Vorlage as the reason for not making a literal translation:
“The Rock (הצור), his work is perfect,
for all his ways are justice.
A God of faithfulness and without iniquity,
just and upright is he. (Deuteronomy 32:4) [ESV]
his works true, and all his ways judgment: God faithful, and there is no unrighteousness; just and holy the Lord. (LXX-Deuteronomy 32:4)
He made him ride on the high places of the land,
and he ate the produce of the field,
and he suckled him with honey out of the rock (מסלע),
and oil out of the flinty rock (צור). (Deuteronomy 32:13)
He brought them up on the strength of the land; he fed them with the fruits of the fields; they sucked honey out of the rock, and oil out of the solid rock (πέτρας).
In verse 4 צור was rendered as works. In verse 13 both סלע, honey out of a rock, and צור, oil out of the flinty rock were rendered with πέτρα. This indicates the meaning of both words was known.
Treating both סלע and צור as the same, πέτρα, is consistent with how the LXX describes the two water from "rock" events after being brought out of Egypt:
Behold, I will stand before you there on the rock (הצור) at Horeb, and you shall strike the rock (הצור), and water shall come out of it, and the people will drink.” And Moses did so, in the sight of the elders of Israel. (Exodus 17:6)
Behold, I stand there before thou, on the rock (πέτρας) in Choreb, and thou shalt smite the rock (πέτραν), and water shall come out from it, and the people shall drink. And Moses did so before the sons of Israel. (LXX-Exodus 17:6)
“Take the staff, and assemble the congregation, you and Aaron your brother, and tell the rock (הסלע) before their eyes to yield its water. So you shall bring water out of the rock (הסלע) for them and give drink to the congregation and their cattle.” (Numbers 20:8)
Take thy rod, and call the assembly, thou and Aaron thy brother, and speak ye to the rock (πέτραν) before them, and it shall give forth its waters; and ye shall bring forth for them water out of the rock (πέτρας), and give drink to the congregation and their cattle.
Clearly the LXX recognizes the meanings yet chooses not to make a distinction.
There is one place in which the the LXX preserved the metaphor:
1 And David spoke to the LORD the words of this song on the day when the Lord delivered him from the hand of all his enemies, and from the hand of Saul. 2 He said, “The LORD is my rock (סלעי) and my fortress and my deliverer, 3 my God, my rock (צורי), in whom I take refuge,
my shield, and the horn of my salvation,
my stronghold and my refuge,
my savior; you save me from violence. (2 Samuel 22)
1 And David spoke to the Lord the words of this song, in the day in which the Lord rescued him out of the hand of all his enemies, and out of the hand of Saul. 2 And the song was thus: O Lord, my rock (πέτρα), and my fortress, and my deliverer 3 my God; he shall be to me my guard, I will trust in him: my protector, and the horn of my salvation, my helper, and my sure refuge; thou shalt save me from the unjust man. (LXX-2 Samuel 22)
Here סלע was rendered as "rock," πέτρα when referring to the LORD; yet צור was not when referring to God. [Additionally, צור is used again later in the song (vv. 22:32, 47) and neither is rendered as rock.] This song is also found in Psalm 18, with minor variations. There, סלע referring to the LORD was rendered as στερέωμά, firmament.
In the lone exception, David calls the Lord "my rock" and is obviously using the term metaphorically. Since the song is introduced as and David spoke to the Lord... it is clear David is speaking to the Lord and not to a rock (i.e. an idol). In this case, the exception preserves David's actual words in a context which cannot be mistakenly understood as David singing to an idol.
While the LXX's use of πέτρα does not distinguish between סלע and צור, the translator preserved the metaphor only for סלע not for צור. The reason for preserving the exception can be attributed to the parallels with that of the second water from the rock episode: Moses did not speak to the rock סלע, David did.
Perhaps in the mind of the translator there is a further distinction intended in that by failing to speak to the rock Moses was denied entry to the Promised Land. On the other hand David sings to the metaphorical "Rock" praising Him for rescuing him from his enemies and keeping him as king. Of course, the translator also knows of God's covenant with David and the restoration of the Davidic kingdom was an expectation of the time.
Finally, the removal of the metaphor in the Psalm can be attributed to its liturgical use. Where David's song is clearly a personal speaking to the Lord, a liturgical use of the Psalm by others fails that test, and so, like all other references which might be mistaken as approving the use of idols, the metaphor was removed.