Could you explain why you think so? Given all these bonusses you mentioned, i am pretty sure that a well prepared defensive line makes up for the planning bonus. I do agree that the attacker has the advantage of the initiative (and that's good), but if we focus on the bonus assigned in game mechanics, I believe he will have smaller ones most of the times. I believe this is supported by the WWW we saw; I remember many instances in which defenders had better combat stats (not the chinese obviously), and only got defeated because the attacker just had more troops or managed to encircle them.
the attacker gets to choose the tempo, they decide where the battle is fought on a defensive line, and they get to concentrate their forces. you certainly don't have to remove a defensive line to defeat it, just to break through at 1 or 2 key points and the defenders are forced to adapt to your attacks, because if they don't you will encircle them and cut their supply off. they are forced to launch attacks of their own to try to close your breakthroughs or they will have to fall back and fight a retreating battle.
on top of that, the longer the defensive line is, the harder it is to have your veteran fast response troops in key locations, you can't defend everywhere with your best troops, but the attacker gets to attack wherever they want with their best troops at the locations they decide at the weakest / have the most favourable terrain.
its more of a strategic advantage than a pure "i have more combat modifiers" though, its slightly more intangible. so when you say that the defenders had better combat stats but got defeated by encirclement, thats the reason why attacking is better, you don't have to overcome a heavily foritified position (like say, the maginot) if you can encircle it.
its harder to visualise with world war wednesday though, its a real mobile warfare idea and both china and japan are fighting with barely any supply and equipment with (nearly, theres some cav and like 2 motorised) infantry armies with basically no air force and no tanks to speak of. if they were of equal quality and both having ample equipment it'd be a real slow grinding blood bath that would favour the defenders attrition wise, you're not wrong on that count either.
