The display itself is round, although the module is a square with trapezoidal corners.
This is odd because you get the intended shape but not the benefit of the shape. There are plenty of displays that are actually round, such as for smart watches.
I'm curious if Google did this for cost and had decided on a larger bezel from the start.
My guess: it's much easier to cut straight lines. If you don't need it, why go through the more complicated step to cut it circularly.