Because they not only shaped out who are ancestors were and how they saw and explained the world, but they also talk about the genealogies of the people and how they are literally the children of the first deities themselves.
For example the Visayan creation myth. Now the myth slightly varies depending on the two Visayan groups the early Spaniards noted like Miguel de Loarca, who noted that there were two separate groups who all referred themselves as Bisaya. They were the people living along the coasts and those in the mountains and within those two groups there was even a couple of subdivisions you can say of each group.
In Loarca’s writings, he wrote down the creation myths that were told to him by the Visayans and how there was a separate but very similar myth between the Visayans living in the mountains and those along the coasts. The people in the coasts, the Yligueynes, have a creation story where there was originally only two deities, Kaptan and Magwayen, and where they came from no one knows. They married and from a reed that was planted by Kaptan came forth the first man and woman, Sikalak and Sikabay. They both married and had 2 children, Sibo the first son, and Samar, the daughter. They also both married and had a daughter name Lupluban, who Loarca says later on in the same manuscript regarding laws on mourning, war, weaving, etc. that their laws were given to the people by Lupluban herself and her son. Then Lupluban married her uncle, Pandaguan, the third and youngest child and second son of Sikalak and Sikabay. They had a son name Anoranor (who is the son I just mentioned that also gave the laws to the people) who also had a son name Panas, with a woman who Loarca didn’t mention the name of at this point. However Panas was said to be the first person who waged war and used weapons (which of course would be explained as the Visayans were described by many as a “warrior like race”) when he waged war on Mangaran because of an inheritance (there isn’t a mention of the relation of Mangaran and who his parents as far as I know of but I’m assuming he was Panas brother or cousin in regards to inheritance). With Pandaguan you get the explanation why people die and don’t come back and with Lupluban you also get the reason for concubinage and thievery.
Which is really interesting because you have this whole genealogy of the first ancestors of the coastal Visayans and explanations of why and how things happened.