You can be on-call and still do other things -- it's not all or nothing. My workplace has tiered SLOs for on-call, some being as relaxed as O(30 minutes) for first response.
Yeah you can do other things but you're never really 'free'. You always have to be able to drop what you're doing at a moments notice. I really hate that feeling.
I'd rather just be there when it's really needed.