You don't have to, and in many of those 30-40 hours courses, especially programming or software-related topics, it is very difficult to record them with high-end cameras for that long. I think the topic may also play a role.
However, I do use my face along with the content for the following reasons:
1. It increases trust. Students bond better.
2. You can make a great first impression with a smile and confident body language.
3. Often, the entire material is not made of ppts, whiteboards, or demos. Sometimes, you may want to take 4-5 mins to explain things. Without your talking head and hand gestures, you will have to put a ppt slide as a still image and a voice behind it for saying the stuff you want to say. You would expect students to stare at the same ppt slide for 5 mins, but the students obviously won't. They are distracted in 30 secs.
4. During the pandemic, everyone got used to the camera-driven meetings and lectures. Pre-covid, I had done so many successful webinars, where the speaker won't appear and would still seem fine. Not any more.
5. It is also a good branding technique if you want yourself to be recognized as an expert beyond your course chapters.
