At the risk of sounding defensive, I have to say that this is pretty much a "welcome to the magical world of video games!" type of a thing.
What Tim said about knocking Wake out of the level or getting stuck somewhere is definitely true, but there's another aspect to this. If Wake died instantly when being hit by a combine harvester (as just about anyone would in real life), people would find that extremely annoying. We could definitely have a longer animation cycle there, with Wake getting knocked down and having to get back up again, but it's not quite as simple as that. You would soon come to hate it, and here's why: if the player can't get back to controlling Wake quickly enough, then the harvester can just keep running over him until Wake dies because the player can't get out of the way, since he's stuck twiddling his thumbs while Wake gets back to his feet. That's basically the same thing as killing Wake instantly, except now the player has to spend time watching Wake die without being able to do anything about it, which would be even
more annoying. So the damage animation is very short by necessity.
But really, I think this is an issue that goes kinda deep into the psyche of video games. Video games just aren't realistic -- they often strive to maintain an
illusion of realism, but generally people in them do things no one could actually do in real life. (Which is true of much of fiction in general, of course!) Wake does get hurt in ways that would kill a normal man. This is absolutely true, but there are very few games with action elements in them that don't have the same problem. I mean, if Wake got shot and stabbed a hundred times and fell off several high ledges, but just used some painkillers whenever that happened, no one would have a problem with it, because that's such a traditional way of taking ridiculous amounts of damage... but it really amounts to the same thing. That's just easier to ignore because we're so used to it.
That said, the actual amount of damage Wake seems to take from these hits in the videos may not be representative of the final values. You should bear in mind that the material you've seen this far isn't from the final build, and we do experiment and tinker with that kind of balancing as long as we can to deliver the best possible experience. Small changes -- like adjusting the damage a certain event does -- can have a huge impact on how the game plays.