From: Vienna, Austria
TBH I think it is relatively plausable that it is difficult to intercept transport planes arriving one by one throughout a day, at low level, trying to use natural obstacles and clouds as cover, with LRCAP from bases > 120nm away. This also might be part of the rationale for the inability to intercept.
Personally I am ok with this limitation, as transports are vulnerable units anyway, in general suffer from high ops losses, and are unable to lift heavy equipment.
More so, I absolutely dislike the thought of the alternative: that it is possible to completely choke air supply simply by stacking LRCAP over an enemy base from bases far more than a hundred miles away. Thats even more unrealistic, and much more harmful to gameplay IMHO, as it would render one of the rather common uses for transport planes in WWII practically impossible.
And finally, your 6th army comparision is severely lacking.
- The Red Army Airforce had operational fighter bases as close as 10nm from Stalingrad but still was unable to interdict transport flights into and out of the pocket completely.
- The air evacuation, except for transport of wounded began much too late in the battle, where attrition of air and ground personell was already over the limit, and SU air superiority over the area practically continuous.
- We are talking about an army of about nearly 300000 men, the equivalent of 15+ divisions in WitP AE. Thats a completely different scale. Still the Luftwaffe evacced 42k troops over the duration of several weeks.
Your opponent is not moving that scale of troops around by air, guaranteed. Live with and think of a counter rather than complain.