Why don't I need a welding mask when I watch a welding video on YouTube?

I instinctively (almost) turn my head around whenever these guys on YouTube grab their welding apparatuses and lay a weld bead.
Because I was taught to do so when I was witness to a real life welder when I was a young rebbel still.
But why is it not necessary to do so, or wear a mask/protective glasses, when I watch it on my laptop or television?