Most free website builders make money by selling ads on your website. For example, if you use the free Wix plan, then they will show ads on every page of your website. This does not look professional.
Outside of that, most website builder software offers limited freemium versions to get customers to try their platform, and they make money when you upgrade to their paid plans.
Only WordPress.org offers the full-featured website builder for free, and since it’s an open-source project, they do not show any ads on your site.