If you're paying attention you'll notice that, typically after cabin beverage service ends, the flight attendants will make and deliver coffee to the cockpit. What happens to that coffee once it's behind the door is, of course, purely speculation.

I see the pilot (or co-pilot) during at least 50% of the flights I take in the US over two hours in flight time. If you've noticed a FA blocking the aisle to the galley with a drink cart and standing posted at attention watching the passengers attentively, that's why. The cockpit door is rarely open more than 15-30 seconds. If either of the cockpit crew actually leaves the cockpit (say to use the restroom), then a FA must take their place and relock the door. This scenario requires a minimum of two FAs, one to watch the passengers / block the aisle, the other to fill-in in the cockpit.

What! Companies still use manned cockpits nowadays? It's a bit like letting a guy into the server room: Pilot error is the cause of 75%[1] plane crashes (Of course I'm kidding, the pilot is probably the cause of non-crashing 99.99992% [2] of the flights).

[1] https://en.wikipedia.org/wiki/Pilot_error

[2] https://curiosity.com/topics/how-do-people-survive-plane-cra...

Possibly non-US ones. Europe is much more relaxed.