Because they don't actually have to do any real work with them.
If they actually did real work, for extended periods, then they would choose the appropriate tool. For some tasks that is a machete, golok, kukri, billhook, or an axe. Historically, the big heavy knife never really showed. Fighting knives were slimmer faster and not much cop for utility work. All specialised to be effective and efficient. That big heavy Busse isn't efficient; great example of blade making though.
So its all about dress and uniform, to show a certain "big man" syndrome. Dandy fun, which isn't anything new, or wrong for that matter. All men are peacocks, always have been. When modern society robs men of the macho culture and life is pretty risk free then men need some kind of outlet. Big Blade syndrome is just one and so long as no one gets hurt then its fine by me. I've carried a sword for a military parade before and loved the theatre of it all.
As I do practice bushcraft, and know my survival skills, plus do a bit of hunting, I would agree that big knife isn't great, but there are large knives that have their use; but its not those as the thread is asking about. For real work I like the right tool, which means several tools that are good at what they do. Not one that is poor at everything. The One Tool Option is fun, but silly.
If you want a blade for defence then carry a sword. Of you want a knife for working utility then its a kitchen knife or something that cuts. Axes and hatchets are for chopping. For EDC utility convenience then a penknife, a multitool, or a small folder will do.