Why have capacitors improved so much over the years?

I can't understand either why manufacturers go through all the trouble of designing a piece of gear & then at the most critical point insert cheap caps.

For one thing, a large manufacturer might pay a dollar for a cap which you might pay ten.

Adding $100 in parts cost is $500 out the door and $1000 in the store.

Many of the improvements effected by the great unwashed may be seen as detrimental by the designer in terms of sonics, reliability, etc.

An engineer, be it an all out, no holds barred or LoFi PoS has a specification of what it to be achieved and a budget to hold. Better caps could result in a flimsy chassis, under-sized heat sinks, lower quality PCB, cheaper switches, etc. etc.

There are plenty of Best of Everything manufacturers and their prices reflect it.

How many Joe HiFi tweaks are verified in engineering terms? Joe may think it better, but most may not.

I can't understand either why manufacturers go through all the trouble of designing a piece of gear & then at the most critical point insert cheap caps.

When design engineers select components/parts for their circuitry, it is their responsibility to assure all components/parts are up to required specification and not to compromise safety and reliability.
Most boutique audiophile capacitor manufacturer cannot provide required documentation for design engineers to consider installing such capacitors in the design.

@widmerpool Absolutely correct.  Even table salt versus Kosher salt.  And even among brands of salt with Diamond Kosher salt generally superior to other brands.

My EAR 890 amp uses cheap electrolytics, downrated from the original build and then, after they started blowing amps, uprated in the current version.  This is an abberation of EAR equipment.  I haven't had a problem and understand that Paravacini designed his equipment to a price point.  He didn't use the top quality parts but he chose parts which would work and produce the sound he sought.   His genius is that he could take more conventional parts and design great sounding equipment at a price point, not cheap but not high end expensive either (besides his commercial product work).  His affordable tube phono preamps are still highly regarded at their price.

Good post @fleschler. Tim's U.S.A. doppelganger was Roger Modjeski of Music Reference and RAM Tubes. Roger argued that the use of costly boutique parts was often unnecessary, as their claimed superior involved matters unrelated to the part's function in a particular circuit application, and therefore provided no sonic benefit. He further argued that some boutique parts actually compromise a circuit's behavior, leading to not just inferior sound quality, but in some cases reliability problems. Music Reference products are known for their superior stability and reliability.

And a second on Paravini's EAR tube pre-amps: After auditioning the EAR 912 pre-amp, Art Dudley stated in his Stereophile review that it was the first pre-amp he had heard which challenged the sound of his Shindo. The 912 was out of his reach (mine too ;-), but the excellent 868 is an over-looked nice little pre. Single-ended and true balanced outputs (two stereo pair of each, the latter via Tim's world-famous transformers), able to drive a 200 ohm load!