It isn’t necessary any more, which is why all Pixels since the 4a have had small bezels all the way around. You literally got the last Pixel with a thick forehead.
As to why they’re not all perfectly symmetrical on all phones… I’ve seen a few answers:
Cost; it’s expensive to shrink the bezel that the display connector is under
Design style
Shrinking all as much as possible, which typically leads to the side bezels being thinner than the top and bottom
It isn’t necessary any more, which is why all Pixels since the 4a have had small bezels all the way around. You literally got the last Pixel with a thick forehead.
As to why they’re not all perfectly symmetrical on all phones… I’ve seen a few answers: