I think that philosophy arose because fighters were seen only as 'interceptors'.
The idea was that as ammunition was limited, the aircraft was ineffective after a few minutes combat, so why keep flying around?
Get back to base and rearm pdq.
Planes would only be launched when an attack was imminent (observers and/ or radar), so there was no need to loiter.
Less fuel = less weight = improved rate of climb, increased maneuverability, etc.
It was only after the fight moved over the Channel that additional fuel was required.
But I'm not even SLF, so could be wrong.