[pseudonymcoward]

This is only idle speculation but:

A web cam resembles an eye staring at you all the time. This makes people feel weird, like something is staring at them. The threat to privacy is right in their face and on a gut level.

That's the reason so many people cover them even when they won't take other basic online privacy precautions.

[Sylos]

Also just in general, people understand what a camera does. It's much harder to understand the implications of abstract "data" going off onto the internet.