Probably most crew would prefer their own eating area anyway - if they were in same area of passengers they wouldn't clearly be off duty - some passengers would want t o ask them questions, talk to them, want things done for them etc and they would have to be polite and engage whether they wanted to or not.
I think most people i n their off duty or break times like t o get away from work, let their hair down, let off steam, gossip etc - and yo u cant do that in front of your customers/clients/passengers
I know in my work, I appreciate my break time in the staff room away from clients.