unkx80: The following was the old contents on why hanezeki is a seki. Unfortunately, I found the presentation confusing, and so I decided to rewrite it.
tderz: captures three stones, but also costs a liberty.
In semeais you prefer to start from the outside, resp. taking away the liberties from the other group.
However the left inside the black group form a local seki, hence White is unable to start there.
Thus White puts herself at a disadvantage to start anywhere else than the preferred place (outside/other liberties).
This disadvantage continues after , as White cannot approach from the outside by a, as it would be suicide and create a life-enabling shape for Black.
White is forced to fill another (own) inside liberty -> b.
tderz: Now, after Black , White is down to atari c,
but so is Black and thus has to remove the 3 white before playing .
tderz: After , White needs to play in order to be able to kill,
however Black is able to play atari and be 1 liberty ahead .
Herman Hiddema: More important is what happens if white does not play in the above diagram!
Herman Hiddema: Black continues by filling a liberty at , then capturing at . See the next diagram...
Herman Hiddema: White has no option but to play , to keep the black group dead. This doesn't help however, as black now plays , putting the white group in atari.
The above diagrams are the true reason that neither player should capture in the original diagram. At the start, both players have a big eye. By capturing you remove your own big eye. See also BigEyeSmallEye
tderz: The position started as here and White had to find a way to survive (play h).
 Blubb is right, my starting diagram is worthless, it must be different.:
tderz: After black becomes necessary, as is .
Black loses by 1 liberty (2 : 3).