Address books, calendars and task list contain private and sensitive data by their nature. We believe that individuals and companies should be able to be in full control of their own data. This means the freedom to choose where and how the data are stored, and freedom in choice of software. CalDAV/CardDAV are open protocols and thus can be used with various server and client software.
Setting up your own CalDAV + CardDAV server such as Radicale is easy. Chances are if you are reading HN, you can do it yourself. Do it.