I agree with you on cables- I really was not making the argument with regards to cables as they really make no difference in any test I've seen (or participated/listened in). There are areas (damping chief among them) that would be time consuming for the manufacturer to implement to the best of their abilities though. Take a look at the inside of high end Beyerdynamic headphones - the cups are empty, leaving room for all sorts of reflections and unwanted resonance that really muddy up the sound. The drivers, on the other hand, were marvelous. A little damping from a local modder had a complete 180 effect on their sound. Similarly, take a look at the Anaxilus mods - especially the newest iteration - some strategically placed foam inside the cups works wonders, both subjectively and on a measurement level. The question could be asked - if these differences are possible and discernible why don't the manufacturers implement them as well? Surely they know the theory? Heck, damping and accounting for reflections and resonance is a big thing among the high end speaker market... The truth is anything they can't build into the cups, drivers, or frame of the headphone is an extra expense - its not something you can take a mold of and make quickly, it would add man hours and manual labor onto the product. The question then becomes is it worth it? Companies with bottom lines often say no - something that will only show up on super niche measurement forums and not be noticed by most casual listeners is not worth the extra outlay that cuts into costs. I recently had my LCD-3's extensively modded (for the better) by an experienced member of the forums here. Upon hearing a modded set, I asked incredulously why Audeze didn't do this in the first place. The answer was that Audeze had a bottom line and tuning them individually like he did would bring it closer to the realm of 009 prices.
As a disclaimer, I am no means an "every aftermarket mod is automatically better" advocate. There are mods out there that don't benefit or even degrade the sound quality of amps, DACs, headphones and speakers. That said, there are a lot of experienced people out there with more time and less focus on profit than the manufacturers who can add to the products in a way that the engineers knew about but were hamstringed from implementing.
I feel like we're getting away from the OP's question, so I'm going to reel myself back in now. I just wanted to combat the notion of "if it was better the manufacturer would have done it" as I have found it to be patently untrue in some cases. "If it was better and cost was no object" would be more accurate, and cost is still an object even in some of the higher end headphones.
That adding damping to headphones improves the sound for you does not mean it's an across the board improvement. It's just a change that you may like, but others including the manufacturer, may not like nor have intended.