1. The courses of the theme were designed to capture those that a typical college/school/university would offer as part of their curriculum. They are not intended for sale online and you cannot complete the same by logging into a computer and answering a quiz of some sort or completing a few lessons online. You need to attend classes physically those classes and they are handled by staff part of a department.
2. LearnDash courses are those provided LearnDash plugin and typically do not have the characteristics required to capture a course taught at a school/college by a professor. They are more geared for sale online without an instructor behind and typically have online quizzes, lessons, topics, certificates etc. to help complete the same online. Since they serve two different purposes (though there may be few common elements), we were forced to go for two types of courses. That helps to keep things simpler for the developer and hopefully for the webmaster too, depending on the target audience.