<human>: I was watching some videos of Zack galifinakis and came across this as a top comment on a video  

for those who can't see, it has a comment being sarcastic, as zack dressed up as a character pretending to be his "brother", clearly as a joke. The most thumbed up comment was in response to a comment saying

"Did anybody notice that he is actually Zach?"
which 60 people liked, as I assume most can tell this is sarcasm.

then now the most thumbed comment; 

"Well done, nothing gets past you. Whilst you're uncovering people's secret lives, keep a lookout for my dead uncle Jim. 6' 1", wavy gray hair, blue eyes, killed in '72 when a big box full of the absolutely fucking obvious hit him on the head."
which 77  people liked.

So as this is an american show, I assume it is american viewers, along with the common stereotype. So I ask why do Americans not understand sarcasm, what is it exactly they don't get.

This isn't intended as being rude, just I want to know. *

Can you condense the text into a few key points?
<bot>: obvious sarcastic comment gets responded with an person taking it seriously proceeding to be a popular comment, why do Americans not get sarcasm?