Timrhu, I don't know why you don't hear differences in power cables and such. It's a big deal on my system and based on posts here at Audiogon, lots of people get good results with swapping these things out.
Now if you said you thought the price of these were not commiserate with performance gained, that would be a value judgement. I agree that many times a system must be pressed to the upper edge before hyper expensive tweaks make any sense.
As for why manufacturers don't add such things. It's for the same reason as other products, money.
Most of the time OE tires on new cars are not the best available, new homes usually don't have the highest quality faucets or appliances. Photocopiers frequently come only partially charged with toner, and that new triple blade shaver has only two spares in the five slots available.
It's smart marketing. When a manufacturer adds one dollar cost to an audio product, the retail goes up $5.00.
A wise manufacturer leaves off the super AC cord, the gold fuses, the designer caps and doesnt include high end interconnects, so the retail price remains attractive.
Frequently, an audio customer enters the store with maximum price already set in their head. Make a product too expensive and you loose that guy.
Add the fact that not everyone believes in or want's those tweaks and those that do, have more fun adding the ones THEY choose when they've recovered from the initial purchase shock.
Certainly a $25.00 fuse is not a make or break deal performance wise. There are systems that likely would not benefit, even at that small price.
In my system and several others in my group, $25.00, for what's gained is a good value. Factor in (if you don't blow a lot of fuses), you could get three to five years of increased listening pleasure and still be able to pull that fuse when you sell.
I like things that add performance to my system that don't cost a lot. It usually doesnt work that way, so I'm grateful for small, reasonable improvements.