Because a diet rich in fiber has been repeatedly found to significantly reduce the chance of early death?
https://www.sciencedirect.com/science/article/pii/S026156142...
https://pubmed.ncbi.nlm.nih.gov/25552267/
https://translational-medicine.biomedcentral.com/articles/10...
(You can find many more)
Literally thousands of studies with one of the clearest results in nutritional science, frankly.