I mean, it’s designed to stop a bullet which delivers much more force than a human being can ever produce during stabbing
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The short answer is because knives are sharp and bullets are blunt.
Kevlar has exception tensile strength for a relatively lightweight fabric. Which means if you were to take a strand of it and pull it would be very hard to break compared to common fabrics like cotton or nylon.
When a pistol bullet hits (Kevlar is generally only rated to stop pistol rounds) a vest it tries to just tear through with its momentum. That woud be like trying to just tear the individual strands apart. That’s very hard to do. Some of them will break, but you layer it a half inch or inch thick or whatever (not sure what the specs on real vests are) and it’s just too much to get through for a blunt Kevlar. Usually.
Stab it with a knife though, and the blade simply cuts the fibers, it doesn’t have to tear them. It’s still going to be harder than stabbing through like an equivalent amount of cotton or whatever, but it’s very possible to simply stab through kevlar.
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