Good question! It was just the "Industry Standard" when I started coding back in 2010. I remember referencing the Email On Acid blog for coding tips then.
Hi Charlotte, looks like you have the whole email wrapped in a 100% width table then a max-width table set at 630px. You have the top blue header and the yellow bar in a 100% table but not the code below that. I put that in a 100% table just like the blue and yellow header and it fixed the issue.
Have a look at this code. It looks normal in Outlook 2016