You can do that to, moving your mouse over the window reveals the scrollbars, as does scrolling. The reason we hide the scrollbars is because scrollbars are no

You can do that to, moving your mouse over the window reveals the scrollbars, as does scrolling. The reason we hide the scrollbars is because scrollbars are not the one and only consideration that we make when designing a page, information density also matters and so we have to weigh these options together and conduct user testing to see what the impact of various design decisions are. Certainly there is a vocal minority of people who are dead set that scrollbars must always be visible, and we can respect their choice, but we also have to optimize our software for the majority of our users who would rather see more content. The balance we went with was if you want to see the scrollbar, you need to perform an action that engages it, which is moving your mouse over the scrollable area, or performing a scroll operation. We have other things too that we have to consider, like until the mouse approaches the scrollbar it appears very thin and when the mouse is within a certain distance it expands to allow it to moved. Will every single user like this approach? No of course not. But based on our user testing, it optimized for metrics that we care about.