Instructors teaching remotely regularly comment that students have their cameras off. As with laptops in the face to face classroom where instructors may have to tell students to close them from time to time, in the remote class instructors may have to tell students to turn their cameras on for particularly parts of the class.
As part of setting expectations, instructors should establish ground rules for cameras. Something like having them on at the start and end of every day, during breakout rooms, in one on one meetings, etc.. The intention being to ensure that students are physically present in the class and are engaging in the activities going on.
Setting those ground rules does mean that instructors will need to follow up on students that keep their cameras off during the designated times. This could mean calling on the student to answer a question, or just asking the student to turn on the camera to engage. Having the ground rule does no good if it is not followed up on.