Why do designers/manufacturers...?

Designers are techies, for most if they can't measure it ,it truly does not exist for them. Their brain will not let
them hear it because the brain can not maintain a stasis
with such a reverse of foundation beliefs.

My elderly music lover brain lets me hear how PC x makes a DRAMATIC change in amp Y over PC z because it is in no danger of stasis instability.

In both cases it has very little to do with ears.

Peter Walker was an outstanding designer for his time.

A large number of audiophiles, to include reviewers, claim to hear noticeable improvements when stock fuses are replaced with "audiophile quality" fuses such as Synergistic Research Quantum Red and Audio Horizon fuses. Assuming the SR and AH fuses are not snake oil, then IMHO designers are remiss for not using them.

Because there cheap.

Have you noticed that many tweaks go in and out of style? For a couple of examples, I can remember when people were knocked out by Mpingo disks and green cd pens. That's not to say that all tweaks are ineffective, but I'm sure some are.