I know it's for keeping water away. I use it for that purpose on my swords. But why isn't it a lubricant? Maybe it's not the best, but it can serve that purpose.
It wasn't intended to be a lubricant. When it "dries" it gets gummy. Quoting Wikipedia[1]
The spray, composed of various hydrocarbons, was originally designed to be used by Convair to protect the outer skin of the Atlas missile from rust and corrosion.[11][12] This outer skin also functioned as the outer wall of the missile's delicate balloon tanks.
Sure, but if you don't know what you are doing, and have a need for lubricating something at home, on a bike, or on a car, you are MUCH better off with a oil, graphite, teflon, or grease.
Sure professionals use WD40 when appropriate. Sure WD40 lubricates great, for seconds to minutes (before the volatilies evaporate off, thus the WD40 smell), less well after that (somewhat gummy residue) and might well stop a creak. But won't lubricate nearly as well as oil or grease or last as long.
It lubricates for a very short time, then most of it evaporates & leaves behind a slightly sticky residue that helps prevent rust. It dissolves most lubricants that would stay in place for longer (greases) and cleans them off of surfaces.
Surely, there is some context where WD-40 is just fine.
I do note that lubrication is actually complex. Soap feels plenty slick in general, but if I try to shave with it, certain soaps give very poor results. So, of course, not everything slick and oily is going to work for lubricating machines.
But when I rub soap on wood screws, it works just fine in that context!
If you take a properly lubricated widget and as part of regular maintenance you lubricate it, it's well lubricated.
If you take a properly lubricated widget and as part of regular maintenance you spray WD-40 on it, it's less lubricated, might be fine for a bit, but it's mostly a cleaner and water displacer. Ideal in some situations for cleaning, before you apply a lubricant. WD40 does leave something behind, maybe better than metal on metal, but it's gummy and pretty far from what I'd consider a lubricant.
Sure in some cases chewing gum, soil, or sand can reduce friction, I wouldn't call them lubricants though.