I think it's a bit like watching TV vs reading a book for a lot of people. When something is on a conveyor belt into your brain, it's easier to let it flow even if the experience is cheaper, whereas playing a game or reading a book requires a significantly extra level of 'participation'.

Of course, it could just be that I'm getting old, the dopamine hits from gaming aren't there as much anymore and that I'd rather watch skilled people at work ;-)

With "Let's Plays", you're there for the commentary, it's not just a quiet walkthrough. The downside of this is that you, as a viewer, have no timely input (episodes are sometimes done in batches and uploaded over a week or two). But.. maybe that's the attraction, back to the days of yore when you would yell at the Jeopardy/Wheel of Fortune contestants for not getting the obvious answer!

Source: I watch LPs constantly while working, it's good background noise and gives me an easy outlet to divert my attention.

I can't answer for parent comment, but one possibility is that youtube (in theory) allows skipping ahead, real games do not.