Why don't ThunderBirthDay calendars have reminders?
ThunderBirthDay doesn't have reminders because it requires some time and thinking, which I haven't had yet. I really hope that I will have it in the future and that the next major version will come with reminders!
The problem is, that calendars which support reminders cannot be read-only (Lightning has to remember somewhere that a reminder has been dismissed!). If a calendar is not read-only, the user will also have the possibility to edit date, title, description, recurrence etc of existing events and to add new ones. ThunderBirthDay events are quite special, though, as they are always recurring yearly without exception, always one day events, always connected to an existing person in the addressbook, etc. To provide a full-featured (non-read-only) calendar, ThunderBirthDay would require a lot more logic than it does in read-only mode. I do have ideas how to solve the problem, but didn't have time to implement them. Hopefully this will be different in the future!