Do tire / tube / valve combinations that don't leak air exist? I'm not clear on why tire pressure decreases over time. I understand that air escapes through the tube. Possibly some leaks through the valve too? In any case, do tire / tube / valve combinations exist that will maintain a constant pressure over time?

Edit: From reading How long should a road tyre hold its pressure? I learn that air leaks through the tube material. I've changed the title in response - it originally applied to values only.
You assume the whole leak is the valve.  Rubber is not air tight.  Most of the leak is through the rubber.

Clearly can create a near airtight valve.  And with enough rubber a near air tight tube.  But it would be heavy.  Just pump your tires once a week.  I see the link said pump daily.  OK, at least once a week.  On my road bike I pump before every ride.