The word always seems to me to be obscenely derogatory and unfair towards donkeys. Other examples include hawk, snake, rat, weasel etc. Blaming non human animals for distinctively human failings is in MHO little more than exceptionalist cope.
Discussion
Don't worry, the donkeys like it
lol, it's not blame it's comparison, and the comparison is to the personality and manner of the person:
a hawk is tenacious and aggressive
a snake is slippery and stealthy
a rat is a gossip and a scavenger
a weasel fights by creating confusion with random reactions, and they are nervous
a rabbit is flighty and easily killed (you can kill them throwing rocks at them)
a chimpanzee is loud and aggressive
a horse is nervous and powerful, but kinda stupid
a dragon is intimidating and a hoarder
a lion is proud and prone to complacency
a mouse is quiet and crafty
a cat is bold but also very crafty and perceptive
a donkey will refuse to move if they aren't sure about what you are trying to ask them, stubborn and ignorant
a crow is crafty and noisy
a magpie likes collecting things and boasting and being ridiculous
just observe the animals, this is the way they behave, so it is used as an analogy, neither blame nor any kind of condemnation of the animal, they is what they are
yeah, not to forget my favourites
a blackbird is nervous and flighty, and makes pretty songs
a jackdaw is noisy and struts around
a rook gathers in groups and acts all superior as they chatter to each other