Same reasons they drink, I should imagine.
Basically this.
It's not identical but it has mostly the same risks. Over-consumption paired with complex tasks and/or judgement etc. I expect MJ is less risky, since there's no culture of "smoke it until you literally pass out" like there is with alcohol.
Full admission: I haven't been high in... call it an even decade? Not due to some issue with it, I simply don't know anybody who smokes or is able to sell me some, but given a joint I'd spark it up happily and resume turning chunks of wood into piles of sawdust and chips with ever prettier things left behind:
I daresay I'm getting pretty good at this, though it's a weird sort of process:
Need to improve the design of my plane so I can spend less time sanding >>>
Have to make clean cuts with a narrow straight kerf in the sole to slot the tote inside snugly >>>
My experimental dovetail saw handle which was made from an earlier plane tote prototype is not suited for this >>>
Draw out an outline and use the coping saw to get the roughed blank cut and begin the initial rounding steps with it >>>
Start using a rasp to knock out a cleaner more comfortable to hold shape >>>
Get caught up plucking out interesting features in the grain and balancing shaping decisions with grain presentation >>>
Realize at some point I lost track of a whole night because it's almost noon >>>
Finish the shaping to my satisfaction and using the saw blade begin cutting a slot for blade and drilling out room for the spine >>>
Tweak the alignment a little and drop guide holes through the relevant parts of the blade mounting points >>>
Select suitable nuts and bolts and such from my little collection and fit them in >>>
Final sanding (60 grit > 100 grit > 320 grit > dampen the surface > 320 grit again > 400 grit + fine file) and oil it up >>>
Actually make the first cuts on the sole of the plane which I needed the saw for in the first place!
The difference weed produces is that it makes things like spending a whole night rasping out features on a tool handle far more likely because fairly boring stuff is just a whole lot more fun when you're high.
Also the biggest issue with any smoked drug and pets is generally going to be due to their much smaller body and less opportunity in many cases to avoid second hand smoke. Direct and still hot smoke from anything is also going to be very irritating and damaging to dog noses, cat ears, cat nictating membrances, and then you can toss on individual breed/specimen effects into the mix.
I've had several cats that made a point to come hang out in your lap if you were toking up, quickly pick up a contact high and devolve into furry purring lumps of derp. No small concidence that these are also the ones most likely to take apart the catnip toy, eat the catnip, and then derpily purr into a lump of fur.
Your methods may vary but as a rule of thumb treat a cat or dog the same as you would a 2 or 3 year old toddler. As funny as it might seem to shotgun a big bong toke at them, they can't properly judge whether or not they even want to do this, much less appropriately handle the effects of said drugs, nor can your pets, smoke somewhere else or let them slip off to a clear and ventilated area first, k?