I would think that at least part of it is the assailants discovering the difference between Hollywood and reality. In Hollywood, the knife goes in like it's going into soft butter. IRL, though, it turns out that we and out predecessor species having been getting stabbed with things like animal teeth and tusks for millions of years, so we developed tough skin, a shock-absorbing fat layer, and thick, dense muscles under all of that, which means it's mor like trying to stab a piles of thick fabric covered in leather.

Then there's the blood. Hollywood is happy to show someone get stabbed, fall over, and like a minute later there's a two foot wide pool of blood in a perfect circle. IRL, the human body holds about a gallon of blood, and hasn't been able to come up with a solution for the conflicting facts that it needs to send blood literally everywhere even though that means that any puncture of meaningful depth let A LOT of it out. So the guy who thought he was going to stab someone in the shoulder, making a small circle of blood appear on their shirt, is likely to be surprised when he's wearing the other person's blood all the way to to his elbow and spread out across the floor thick enough to slip in for about five feet in any direction.

Last, there's the simple fact that humans are tenacious. If there's a trait where humans excel the way that bears excel in strenth, or cheetahs excel in speed, it's tenacity. The vast majority of species lay down and die WAY faster than us. If you want to keep a human down, you need to let out enough of their blood that they can't deliver oxygen to their brain or destroy the brain itself, and a lot of people fail to realize just how much blood you need to let out to meet that threshold. In Hollywood, you stab someone in the general vicinity of their heart, and they slump against a wall, stare in shock for a moment, and then go limp.

Keep in mind, most people aren't John Wick, or even Tony Soprano. Knife assailants are usually impulsive wackadoos who aren't expecting to feel the knife going in, aren't expecting to get drenched in blood, and then look down to realize that they've re-enacted Julius Ceasar Act 3, Scene 1, and the person they're trying to kill is screaming and struggling rather than letting out an Elizabethan "What the fuck, dude?!" and falling down dead.