So over the years I've read both here and on theSamba that the valves, guides, seats that come with AA and AMC heads are "junk". Why is that? Aren't they all made from hardened steel? Is there a difference between stainless steel valves? Is it a tolerance or material issue?

Is there a specific manufacturer of hardware that head builder use when building heads?

I have a set of AMC reworked heads done 10 years ago by DPR, closed his shop and sold his tooling to FAT, 40k miles so far, used Jakes cam kit, the combustion chamber and spark plug location changes for 2.0 conversion and stainless valves for heat dissipation without the expense of sodium filled. He changed the seats, valves, springs, collars and studs. I still have a bag of the unused studs which are hardened.
I have been very happy with the results.
Remember the aftermarket heads, at least the AMC are for bus motors.

For either the AA 914 2.0 heads or the AMC heads the hardware is of a lower quality, yes. People do get some mileage out of using them straight out of the box but upgrading to high quality valves, springs, keepers etc is worth unless you like working on your engine.
Had almost 60k on my Raby Camper Special heads when I sold the Westy and the 914 is running fantastic on LEN reworked AA 914 2.0 heads with combustion chamber coatings.