Why are 2/3 height bricks with studs on the side 2 studs deep?
In the case of 87609, the piece was first used in 2010 as part of the grill/bumper assembly for vehicles that were the standard 6 studs wide. Its length would appear to be a result of its original purpose. Its width also allows for attaching two rows of detail:





99206 showed up first in 2012. Its design allows for a more compact construction in situations like this:



If the piece were only 1x2, it would have to attach to another part above or below it. The extension lets you attach it flush from behind.