My guess is the mouse wheel made them less necessary to the point where form over function folks won out.

Though I do still see a scroll bars in Firefox and steam. Some auto hide when the mouse isn't hovering over the pane they scroll, but they are there. Similar on my windows work machine.

Might be more of a phone thing where accelerated swipe scrolling is a thing. And because they decided to get rid of the back button on the pixel, they made interactions with the side of the screen mean back, which would make interacting with scroll bars annoying (though I'd rather have a fucking back button and not have that annoying behaviour when I do a movement that looks like swiping from the edge, which happens a lot in chess whenever I want to move a piece on the edge of the board.