First, because most statistics are based on self reported user agent metrics from the top websites. But my parents don't go to Reddit, PornHub, YouTube or LinkedIn, etc.

Second, culturally speaking, I don't think we have accurate reporting of non-western websites. I bet most of China, India and Russia websites don't care about reporting their metrics publicly.

And last, internet usage distribution is very skewed. I myself probably generate 1000x more internet traffic as my parents, so I probably count as 1000x more market share than them. Yet, in terms of "users", I would count the same as they do.