It may temporarily act like a lubricant, but we typically expect lubricants to last longer than a month or so, and WD-40 does not. It's meant as a solvent, and can be followed up with an actual lubricant.
But you are aware and know that you have to reapply KY every time you want to deal with some ... em... tight situations. And after you're done, you intentionally wash it off, because you don't want those places to be lubricated during non-active times.
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).
The point is, different lubricants have a different effective time.
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.
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?