I believe this is Simak's "The Answers" (1954), originally published in Future Science Fiction as "... And the Truth Shall Make You Free." If the Internet Archive is working you might be able to read it in the collection Strangers in the Universe or the original publication.
Four explorers, a Human, a Dog, a Spider, and a Globe, land on a world and find the last traces of a breakaway group of humanity lost for over a hundred thousand years:
THEY knew it when they stepped out of the ship and saw it. There was, of course, no way that they could have known it, or have been sure they knew it, for there was no way to know what one might be looking for. Yet they did know it for what it was, and three of them stood and looked at it and the fourth one floated and looked at it. And each of them, in his brain or heart or intuition, whatever you may name it, knew deep inside himself a strange conviction that here finally was the resting place, or one of the resting places, of that legendary fragment of the human race that millennia before had broken free of the chains of ordinary humans to make their way into the darkness of the outer galaxy.
And yet there is no towering civilization here, or even the obvious traces of one:
The Dog was kind about it. "It's strange," he said. "There is no
evidence of any great development. No hint of anything unusual. In
fact, you might guess that they had retrogressed. There are no great
engines, no hint of any mechanical ability."
The Human, David, decides to remain behind when the others continue on in their explorations, and comes to discover the descendants of those people. He contemplates their simple lives:
It must be great, he thought, this truth of theirs. It must be powerful and imagination-snaring and all-answering to send them back like this, to separate them from the striving of the galaxy and send them back to this pastoral life of achieved tranquillity in this alien valley, to make them grub the soil for food and cut the trees for warmth, to make them content with the little that they have.
To get along with that little, they must have much of something else, some deep inner conviction, some mystic inner knowledge that has spelled out to them a meaning to their lives, to the mere fact and living of their lives, that no one else can have.
David asks for, and receives permission to view the Truth that these people have discovered, only to discover that a computer (big, but not so monumental as Deep Thought) revealed it:
He swung the door wide open and stepped to one side so that David might walk in ahead of him. The place was one large room and it was neat and orderly. There was some dust, but not very much. Half the room was filled to three quarters of its height with a machine that gleamed in the dull light that came from some source high in the roof.
"This is our machine," said Jed.
And so it was gadgetry, after all. It was another machine, perhaps a cleverer and sleeker machine, but it was still a gadget and the human race were still gadgeteers.
The Truth is finally revealed to him:
He led the way to a table that stood in front of one panel of the
great machine. There were two tapes upon the table, lying side
by side. The tapes were covered by some sort of transparent preservative.
"The first question," said Jed, "was this: 'What is the purpose of the universe?' Now read the top tape, for that is the answer."
David bent above the table and the answer was upon the tape:
The universe has no purpose. The universe just happened.
"And the second question..." said Jed, but there was no need
for him to finish, for what the question had been was implicit in the wording of the second tape:
Life has no significance. Life is an accident.
"And that," said Jed, "is the Truth we found. That is why we are
a simple people."
