Their designers stripped away everything that was unnecessary, and allowed the content to become the design. It's a very difficult aesthetic to pull off because it relies entirely on the most challenging aspects of UI design: spacing, grids, and typography.
At first, I didn't like it either, but over time have come to love it (and have become inspired by it in my own designs).
Alas, it is possible to strip too much and I think Google is guilty of that.
> aspects of UI design: spacing, grids, and typography
I'd say these are more aspects of content design, albeit still very important in UI, but less so, for a simple reason: typically there is less content.
You still have to have some visual hierarchy, make important things prominent and less important less so, mark interactive elements as such. When you strip too much you are forced to use whatever left for that and it can be a stretch, like excessive use of white space.
At first, I didn't like it either, but over time have come to love it (and have become inspired by it in my own designs).