I have read articles explaining the benefits of using semantic tags like section, article, nav, etc., so I am aware of semantic HTML. On paper, everything makes sense: improved SEO and accessibility.

But after that, I wanted to look at a real-world example of how a website was actually implemented. I was shocked to see that these tags are not used when I looked at Facebook, Netflix, and Google. Instead, it is simply a collection of nested divs, which every guide advises against using. On Twitter, things are a little better, although some tags—for instance, section—remain absent.

No se usan porque fueron implementadas hace poco en la versión de HTML5, y la mayoría de paginas que mencionas son creadas con versiones anteriores, si deciden cambiar las etiquetas les llevara un trabajo muy grande.