The plain answer is that they have no reason to.
If most of the lesson is the teacher talking and the class listening, with the occasional cold call, would you feel any need to turn your camera on? Most people will kill the camera, mute the mic, and unmute only when they have to answer. Nobody wants to be looked at for two hours when they will speak three times.
So give them a reason and the right conditions.
In English classes run on the Cambridge CELTA model, learners get a lot of opportunities to interact with each other, closely supervised by the teacher. Online classes are no exception: people go into Breakout Rooms to share experiences in English, work through exercises and do activities tied to the lesson.
Why Breakout Rooms work
Fewer people. Showing your face to a room of thirty or forty feels exposing. Talking to one or two classmates does not.
It is odd not to. Nobody wants to converse with a motionless avatar that occasionally emits sound.
No one is marking you. Without the teacher constantly present, people relax and share what they actually think.
Activity design matters just as much
If you suddenly ask a class to discuss the strengths and weaknesses of going to university, it is hard to know where to start. Hand them a list of strengths and weaknesses and ask them to rank it from most to least important, and the same discussion becomes lively.
In the end, a class works or fails on both sides. The teacher's job is to design activities that genuinely suit discussion and to build an environment where people feel comfortable. Do that, and learners do not just switch the camera on, they start building the lesson with you.