Totally agree... defense should be easier AND cheaper and: IT IS in this game- HURRAY!
A few premises follow once you are attacking someone:
1) You have more resources to spend doing this and so have that advantage. (if you don't, what are you doing!?!? Better have a GREAT tactical plan, because LUCK isn't one of your soldiers!)
2) You are picking the battlefields and the troops that will show up (if you're not, you're not a very good offensive general and ought to stop trying).
3) It is taking place in THEIR country with you ravaging the countryside and ruining their potential for gathering resources (if you're not- again, you suck).
The only offensive actions I've taken when the above has NOT applied have been at sea with Privateers, and on minor counter-attacks, insurgencies into their terrain when I can manage it, in order to disturb an enemy at MY gates (make them think twice about being here)- but not to actually take over their cities (unless the opportunity presents itself) more to just penetrate his country as deeply as I can with my units blowing up mines and salting farmland and drawing off his offensive units. But even this is best done across mountain ranges with infantry, not expensive tanks unless you have a wide open plain or desert you need to cross in order to even GET to their terrain and you don't have a navy to transport them.
Oh, the only other offensive action I've taken which disobeys the above three rules is -ahem- nuking a Chinese city just to see the special effects and because the British were going to win anyway! Disappointing- we NEED a mushroom cloud and more flashing lights man!