There are many potential reasons for this, but I think the biggest ones are mainly psychological.
Do you wear those cords to hold your glasses? Why not? Any older person might tell you they're super useful.
I suspect there's something similar here.
Moving on, that's just not how wizards think. Many very simple mechanical devices in use in the muggle world completely puzzle wizards, simply because a wizard has become conditioned to trust magic first. It's not in the mind of a wizard to use a device (ugh!) to hold a wand to their wrist. They might use some magical charm, but if they did, we wouldn't see it, or we wouldn't know it when we did. And this could easily explain why a disarming spell seems to throw people around from time-to-time... it's interacting with their wand protection charms.
But I don't think people went around every day expecting someone to jump out from behind a bush yelling, "Expelliarmus!" Voldemort and his followers use far worse things when they come for you. Really, I have the impression hardly anyone used it outside of sport dueling, until Harry — being something of a celebrity — popularized it. Thus, it's not something one would normally think about much at all, let alone take the fore-sight to prepare for. Even Voldemort's followers, in preparing for the Battle of Hogwarts and aware of Harry's tendancies, are likely more concerned with worse curses that directly attack them physically: Stupefy, body/leg-binding, etc.
Inasmuch as the wizarding world has a military, I wouldn't be surprised to find some kind of device in use. If things had become worse, more people might have adopted something along these lines. But part of the story is Voldemort deliberately avoided real military-style assaults before the Battle of Hogwarts. Rather he worked covertly, taking power behind-the-scenes to avoid that kind of direct confrontation. The battle was to be sort of his coming-out party. Even at the point where our heroes find themselves about to enter combat, these devices may not have been ready to hand and of little use against opponents willing to throw out Avada Kedavera at the drop of a hat.
One must also remember the nature of wands, that they have some will of their own. It's conceivable use of such a device would in some way also magically bind the wand in a way that limits the wand's utility.
There is also the concern magic is, well, magical. Could a purely physical device acting in this capacity hold up, or would it be easily undone by the magic? You could, perhaps, magically protect such a device, but again it seems like the point of Expelliarmus is to get past that kind of protection.