Because if you want to cover all possible lasers you'll block out the visible spectrum as well and won't see anything.

You can get some overlap tho. I have 520nm goggles that tone down 465nm.

That's mostly because it's tough to get a perfect notch filter in the visible spectrum but you'd never want to use the 520nm with a 465nm unless it was low enough power the fuzzy edge of the filter knocked it's power down enough to be safe.

Then you can't see anything.

They're narrowband filters. A welding mask would be a wideband filter, but is much harder to work with when it's engaged.