I just finished the outline, cutouts and mounting holes for a box. It took me some time, and got the idea that I might share it with others so they don’t have to draw it themselves.
But what if we all contributed to the same library when we share our enclosure PCB designs? The principle is the same as for a component footprint. Not sure if it should be shared as a footprint or a pcb that you append, though. For those who only use extruded aluminum get by with only making a square, but for hammond boxes and non-standard designs like altoid tin cans you can spend at least some time designing it from scratch. And if your box has no drawing, like the one I just did, then you have to take your caliper out and really start spending some time.