There are people who look for short terms courses, and just want to grab a certificate of completion. If the skills they want to learn, can be learnt in 30 minutes, that is fine!
However, all the learners are not looking for shorter course and/or a certificate of completion. There are masterclasses of more than 10 hrs. duration, on lots of different skills (technical skills, programming languages, business skills, certification courses,graphic tools etc.); these masterclasses cannot be completed within 30 mins. So, it is purely based on the instructors what they want to teach, and who all are interested in their course curriculum. Udemy has just put a minimal condition for launching courses as they cater to diverse group of audience.