Why do designers/manufacturers...?

A good manufacturer is going to put the best (within their design/price point) equipment into/on their units. many of the "tweek" out there are supposition and unproven and fall within the category of word or mouth or one's opinion vs another's. Chassis isolation is incorporated in many high end products. The designer is going to install power cords that meet the minimum + standard for operation of that unit. If they have the time, money and inclination, they may try other's or other devices to see/hear how they sound and if available in sufficient quantities, by reputable manufacturers (that will be around in the long term), they may use it. resonance control, chassis isolation, transformer isolation, proper grounding schemes, properly sized conductors and power cords are not tweeks, they are proper engineering.

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A different way to look at it is what "tweaks" do designers do with their designs that are built in already, might not be readily apparent or even advertised, and that potentially could render certain additional external tweaks irrelevant or insignificant?

That's why YMMV, especially when it comes to things like tweaks.

Are we saying audiophile grade fuses are snake oil?

Certain capacitors and resistors made by various manufacturers have been shown to actually make the "sound" better. For the Engineer, as long as the specifications for that capacitor meet their design requirements, they are good-to-go. However, if they have the time and knowledge, they would try other cap manufacturers as is the case now a days and either place them in their product initially or "upgrade" to them later. It is now science, not snake oil. It is a known fact that certain capacitors and resistors are simply better in the sound of the equipment. But, that wasn't always the case. So, I have no idea if audiophile grade fuses are snake oil or not, as an Engineer, I can't see how that would make the slightest difference in the sound coming out of the equipment if it meets the same specifications as any other fuse. But, the same could have been said for capacitors and resistors. Other than percent tolerance, actual resistance value, power rating, what other measurement was there? But, it turned out that some really did affect the sound. So, maybe "audiophile grade fuses" can do the same. Many, Many years ago, I didn't believe that interconnect cables really affected the sound. Boy was I surprised on blind test and A/B comparisons on equipment to hear differences. I swapped "higher quality" cables in my system and yes, there is a noticeable difference in sound. So, for fuses, mayhap that just hasn't gotten to the various equipment manufacturers yet. Give it time. It is really does affect the sound to such an extent that is should be incorporated in the final design/manufacture, it will be.

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Just to follow up. As an Engineer, I knew that transmission lines, cables and wires, have certain impedance characteristics over frequency. They all have a certain resistance, inductance and capacitance, which I knew constituted the "impedance" and varied over frequency. So I "knew" inherently that different interconnect cables "should" affect the sound differently. I just didn't believe it until I heard it. But Fuses? How can a fuse have a different impedance characteristic over other fuses? you know, it is still a wire. A short wire, but a wire, with internal terminals, and is made out of different materials, so, yes, it could affect the signal. To the extent that it justifies spending a ridiculous amount for one fuse vs another? I'm not sure. But I would love to try it out and see/hear.

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