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