Because by using a custom domain, it means that you have to rent and not buy your own domain that you have to pay the rent. This is usually the reason why many people choose to not use a custom domain. However, it is actually better to use a custom domain. Why?
The first thing to know is that the domain is not the same as hosting. Hosting is the place that the SEO provider of yours, such as www.pagesatu.com, will rent on a particular server to put all that is needed so that our website or blog can run. While the domain is the name or address that you can call through the browser to open the files that are in our hosting it earlier.
Of course, it is not that simple but that is the outline. If you use a free blogging service kind of blogspot.com which automatically gives also a free sub domain for you, it means you do not have to pay for hosting because it is provided free by Google as the blog service providers that earlier.