Maybe you think it's not a _good_ lubricant? But a lot of its use is to extract stuck things which requires a thin lubricant
The word you are looking for is “penetrating oil”. The problem with a too broad definition of “lubricant” is that I wouldn’t use Coca Cola to lube my engine or KY Jelly to lube my bike chain. WD-40 is a shitty lubricant for metal-on-metal moving parts, so it’s helpful in that context to say that it isn’t a lubricant.
Neither does KY and yet here we are.
While WD40 wasn't designed as a lubricant it does function as one.
People using heavy equipment often grease their machines daily but no one claims grease isn't a lubricant.
I have to put bar oil in my chain saw every time I use it, and often have to refill it during the day. Is that not a lubricant?
But a bike chain is a thing that you want lubricated for a long time, and not have to think about lube every time you want to use it (well.. in a normal, bike-driving way). Is it still lubed? Did it rain the last time you drove a bike? When was the last time you lubed it? Some other lubricant would last much longer and lubricate much better, and with a bike chain, lasting many months is good thing.
That's why we have and use different kind of lubricants for different things, and WD40 is not a "good choice" for most of the stuff some people use it for (like bike chains... or anal).
I'll see myself out.
As I said: a chainsaw is constantly adding lubricant to the bar. Meanwhile the swing I built for my son had white lithium sprayed on it and still doesn't make a sound, after full exposure to 3 monsoonal wet seasons.