Ask HN: Why css frameworks?

Over the last couple of months I have seen an influx of new CSS frameworks.

My question is why: why? Why go through the effort of learning and implementing a framework when vanilla CSS works so well. I see that a lot of developers are pushing for sites to load faster, be simpler, and be smaller. Yet at the same time we are bloating pages with these frameworks for stuff that could be achieved quite easily with "normal" CSS. What are some of the benefits of using a framework and why do so many people choose to use one?