This is true today, almost certainly for reasons related to economics and manufacturing. It was not true when those chains were initially (1) specified, and (2) produced.

Given the same internal width, a narrower chain will generally work on a system originally designed for a wider one provided it's not narrow enough to jam between cogs or chainrings. (Chains for 6-, 7-, and 8-speeds have always had the same internal width.) Specifically, a 7.1mm or 7.3mm external width chain - the typical external width of today's 6/7/8 speed chains - will generally work acceptably on 6-, 7-, and 8-speed drivetrains, as well as most vintage 5-speed drivetrains.

Because of that fact, manufacturers today simply don't bother to manufacture and market 3 separate chains for this older segment of the cycling market. It's far cheaper to manufacture/distribute/market one product than 3. Separate tooling, packaging, and distribution costs are not insignificant compared to a those for a single product.

I have read (but haven't personally experienced) that some older drivetrains - e.g., some older 5-speed and perhaps some 6-speed - actually need wider chains for proper shifting. That's the only reason I can see why 6 speed chains (like the KMC Z6 and a current 5/6-speed offering from Giant available thru Performance Bikes) are still in production, or why the KMC Z33 was produced up until circa 2020.