With a ~very~ few distinct cultural exceptions, the knife has never been a preferred weapon among humans who had to fight. Especially not among those who fought regularly and often. Human primary weapons are the spear, the bow, the sword, the pike, the rifle. These are the weapons that come up again and again when combat happens. Distance weapons. Long, back off, fuck you, die over there weapons. Those were what professionals used. If a professional used a knife, it was because their primary weapon was unavailable or rendered useless. A knife was a weapon of those already in the cacky. A soldier using the knife happened when shit already hit the fan.
Exceptions? yes. The assassin with the stiletto? They're not there to fight. They're there to kill, and then leave. Ideally, fighting won't happen.
The knife does have it's place of supremacy, though. It is humanity's penultimate tool. I don't care how ass-backwards you are. Even those who didn't figure out a wheel or fire learned quick that a sharp edge was a damn handy thing to have around.
And it's those tool users I'm thinking of when I rack my brain trying to think of a knife as a primary weapon. The Pukko of Finland? Tool. A Gurkha's Khukri? Tool. The fact that they're devastatingly effective weapons doesn't stop them from being tools first and tools more often.
This tool/weapon divide also explains something else I've been unconsciously doing: NOT using a reverse grip. It's a grip and movement only used in fighting with a knife. Nobody chops vegetables for dinner with a reverse grip on the knife. I don't sit down and eat a steak using a reverse grip. Old-timers don't sit on the porch whittling old sticks in a reverse grip. It's first and foremost a fighting grip, which makes it feel a bit unnatural in the hand.
Which brings up another thought: what's the difference between a knife fighter and someone who finds themselves in a fight and happens to have a knife? Which do we see more of onstage? On screen? On the page? on the news? on the blotter?
There's some more pondering to do. This could get interesting.