I have to agree. The scarcity of iron in Civ games, especially Civ6, makes absolutely no sense. During the bronze age, major of centers of power grew up around the availability of copper, tin, and arsenic (which were often found in different places); in many cases these centers of power collapsed or diminished after the transition to the much-more-readily-available iron. Civ seems to assume bronze can be found anywhere while iron needs careful sourcing, but historically it was definitely the other way around--it would make much more sense to make copper and tin strategic resources and iron a bonus resource (suggesting not the presence but a particular abundance of iron).
The problem was that the sources of the metals for Bronze the Alloy, especially Tin, were nowhere near the sites of the Civilizations in the Middle East, Indus River valley, and Mediterranean. This stimulated long distance trade but also made Bronze a relatively rare and expensive metal. Iron, by contrast, was available in the limited quantities required (30 kg of iron is more than enough to equip and armor a swordsman, 100 tons of iron will equip an entire Roman legion head to foot) almost everywhere. Notice that the armies of the Bronze Age tended to be a few armored Nobles and a bunch of non-metallic followers (see The Iliad), while the Iron Age saw mass armies carrying metal weapons and wearing metal armor (Roman Legions, Greek Hoplites, etc.)
Civ's perennial problem is that it makes no distinction between the limited quantities required for effect in the ancient era and the Industrial Quantities required in the Industrial Era and on. You might find enough Copper to make jewelry or bronze for a band of warriors in many places, but to find the 100s of tons needed to wire one industrial city for electricity is much, much harder. Iron even more so: 100 tons might equip an entire Roman legion, but it will barely provide rails for a single mile of railroad track, and one medium-sized Ironclad will require 2-3000 tons of the stuff! You could equip the entire Roman Imperial Army of 35+ legions (historically) with the iron ore required to build one dreadnought Battleship. The problem of Resource Supply becomes an Order of Magnitude more difficult, and is entirely missing from the game, leading to absurdities.
Also, as I have posted elsewhere (several times!) the distinction between Bonus, Strategic and Luxury/Amenity Resources is entirely artificial: the use of a resource varied with the requirements of the civ and the technology available. Copper, for instance, may be useful in ancient/classical eras for jewelry, tool, and bronze, but it was also a requirement for efficient electrical wiring in the Industrial/Modern Eras - a Bonus and 'Luxury' Resource (see the effect of electricity in the individual home on Society in the late 19th, early 20th centuries). Gold is the classic 'Luxury' Resource, but it and Silver are also the basic metals for Coinage, therefore required for markets and most trade, and in the Information Era gold becomes a requirement for Solid-State electrical connectors, making it a Strategic Resource (computers, electronics in ships, planes and weapons) and Luxury/Bonus Resource (personal computers, cell phones, tablets - both Productivity and Happiness making).
Every Resource should simply be a Resource, with its application left to your situation/technology in the game, and with Substitutes available or most of them: Oil for Latex, Silver or Paper Money for Gold, Obsidian (makes very sharp blades) for Bronze or Copper Tools (or Weapons - see Aztec Eagle/Jaguar Knights). Then make many of them available in either Basic or Industrial quantities, revealed as you get the technology to find them, and the Resource system would be much more dynamic and interesting throughout the game than it is now.
