Why is it that Beerus and Whis know Frieza but not Buu? It's a known fact that Beerus was asleep for 38/39 years.

When he wakes up, Whis tells him that a Saiyan defeated Frieza.

I know 39 years is a long time however Buu was also on rampage throughout the universe way before that (fighting the Supreme Kaihous and so on).

How come they know about Frieza and don't know about Buu?
In Dragon Ball Super, as well as Battle of the Gods, Beerus did not know about Majin Buu. The manga has not given a definitive answer as to why they don't know each other, but we can narrow down the potential answers.

In an interview, Akira Toriyama has stated that contrary to Supreme Kai's statement about Bibidi creating Buu, Buu has actually been around since the beginning of time. Bibidi just knew how to wake up Buu from his long slumber.

We also know that gods aren't born as gods in Dragon Ball. They obtain the god title after training and fulfilling their duties. Thus we know that Buu existed before Beerus became a god.

This means that Beerus not knowing Buu can be attributed to any of the possibilities below -


Beerus and Buu never met each other - possible due to the excessive amount of time they spend asleep.
Beerus might have met Buu, but in kid Buu form. He has never seen Mr. Buu before.
There was no reason to meet. Beerus destroys planets to maintain the balance of the universe. With Buu also destroying planets, he essentially did Beerus' job for him.


We also know that Beerus sealed Old Supreme Kai into that sword. During that time, Buu was also asleep, so there was no chance for Beerus to meet Buu during that time. 

We should also note that Old Kai described Buu as more evil than Beerus while Beerus was more powerful than Buu. It's possible that Beerus didn't need to seek Buu for a challenge if he was already stronger than him.