I find this subject very interesting, thank you for sharing (ha!) your thoughts, OP.
I think our cultural expectations around sharing place expectations on children we wouldn't place on adults. Like you, OP, I often think that if I was working on my laptop in a cafe and someone came in wanting a go, and someone else told me to share, I wouldn’t be very impressed. And I would rightly think it was bonkers.
But where I live, it’s absolutely the norm, and I really struggle with not wanting to force DD to share, but worrying that people will think I’m rude!
Second, I think parents do a lot of telling kids to share (by barking ‘share!’) but without much explaining of how. It needs teaching, ‘like, you take the bucket and X can take the shovel, and together you can fill the bucket with sand’. Or whatever.
And thirdly, I also heard (on a podcast called Your Parenting Mojo, for anyone interested) that our cultural expectations are sharing are really specific to Western culture now. It’s not universal. And they also said that developmentally, genuine sharing is cognitively almost impossible for little kids. They don't have enough understanding of ownership and the passage of time to really be into turn taking or anything.
I don’t know where that leaves us. But it’s all very interesting.