Why is that? If you wear your PPE (helmet with appropriate shade while welding, face shield while grinding), I would think you would be protected.

You largely are, but incremental damage does happen through error largely.

Examples:

Difficult to see work becomes root cause if phase error between protection in place and start of arc.

Ever glance over and see the bright arc? Hard to not do that.

Use of marginal protection happens for a ton of reasons.

Each flash exposure adds up.

Yeah, and my experience with welders (used to supervise shipyard jobs on drilling rigs) is that a lot of them are pretty bad with eye protection while using cutting torches as well.

Oh good call. Those are well above safe limits. I see this all the time myself.

Yep - this largely what I've been told. Incidental flashes and cumulative exposure.

Less so your vision and more your lungs.