Because people want a desktop application experience inside of the browser.

When I was playing a Counter Strike web app example posted here a few months ago, it was endlessly frustrating to have Control and W mapped to actions that you may want to perform simultaneously while also not being able to suppress default behavior. I kept accidentally closing the tab as a result...

I personally think that the ability to override default shortcuts isn't necessarily a bad thing, but I definitely think it should be opt-in, like capturing the mouse cursor is. There should be strict separation between "app" features and what a default website should be able to do. Overriding shortcuts should be one of the "app-only" features imo.