Starting your own blog and building an audience is difficult, but you get to decide on the layout, design, branding, URL, topics, ad placements, etc., but if you write for someone else's blog you definitely don't keep all the money, and you probably don't have a lot of say over what ads to run, where they're placed, the name of your blog, the design, etc. They write not because they love the topic (because if they did, wouldn't they have already started their own blog?) but because they could make $50, $100, maybe a few hundred dollars per month.