How Conductive Are The Metals Used To Make Electrical Fittings?

There's more than conductivity to consider.
Durability
Raw material cost and availability
Ease of manufacture of components,,,

Interesting. I thought brass would have been much more conductive than that.

Some applications need to be hard wearing, others need to be springy, also issues of corrosion resistance, etc. Conductivity is not the sole factor, neither is price.

Also alloys are not just a mix of metals, in many cases they form intermetallic compounds and that can influence both physical properties like strength as well as electrical properties, so I'm not that surprised to see alloys lower than their constituent parts in conductivity (copper, zinc, and brass as shown).

As per all the above.
The selection of materials for a product design generally has to balance several opposing considerations, and compromises need to be made, so you would not expect to choose a material on just one or two properties alone. Just an example of how you can't pick a material that's 'good' at everything.

Then there's cost, availability, compatibility, safety, environmental, and regulatory issues to comply with!
There are text books on this stuff!

In many electrical applications, the mechanical properties are much more important than the electrical ones. Copper and aluminium, two go-to metals for cable conductors where conductivity is the key parameter, are too soft for manufacturing many types of components, and aluminium has its specific problems with oxide formation. Hence the many copper alloys, brasses, bronzes being used for terminals etc. Even zinc-plated steel is often quite conductive enough for many jobs and is both cheaper and stronger than many non-ferrous alternatives.

A particularly important function is that of switch contacts, and many different specialised alloys are used for different switching applications. The contacts in a light switch are different to those in one designed for audio switching. The audio switch would soon weld if switching even 100W of mains, while the light switch would develop a high and unstable contact resistance if not wetted by a minimum current & voltage. In one product I made, I noted that the relays and switches I specified had five different materials for their contacts.