Evasion is assumed in karate kata

All arts necessarily make assumptions. To have a starting point you have to have presuppositions. Karate presupposes that the practitioner has the sense to step out of the way of a punch and put his hands up. But then that evasion is not really practiced much as a skill in class. Some beginners might have the sense to duck out of the way of an attack, but then they get so tangled up in the teaching system - the 'front stance, downblock, counterpunch' routines - that they drill that common sense evasion right out of themselves. They hope to develop some higher skill but all along they are forgetting about the assumed factor - the starting point - evasion first.

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Example - ippon kumite (one step sparring exercises) - mid-level lunge punch countered by downblock-reverse punch. A lot of modern karate systems that do one-step engagements  begin with something like this. It is basically a bunkai - a piece of a kata done with a partner.
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Have you ever considered how incredibly hard this first one-step is to do? Your feet get all tangled up with uke's. You have to be faster in order to get your downblock into position at the right time. You bang up your arm against his punch (requiring a lot of 'arm toughening' practice), and worst of all you have to know what is coming to be able to do the move properly.

To be fair, there is some evasion and angulation going on in several of the onesteps above, but Kanazawa seems to be about as good as it gets in the Shotokan world. Also, he's not doing that inane downblock-reverse punch engagement stepping forward into uke - but go with me and you'll see where I'm coming from here.)

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If uke is off on his aim, say he throws a face punch or a reverse or hook punch instead of a low lunge punch, then your downblock misses and you take a hit and look stupid (think Jim Carrey's karate skit here). This supposedly basic one-step is really quite complicated the way it is done. You have to be quite good to pull it off spontaneously. I've seen some TSD 4th degree black belts who could sometimes do it, but I don't think I've seen many others that could do this thing out of the blue.

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For another, perhaps better example, take Taikyoku shodan, said by Funakoshi to be the universal exercise.  Try to come up with some bunkai for the first sequence: left turn 90 degrees into front stance, left down block, step forward right front stance lunge punch.  The first basic application is, "there's a guy attacking from your left so you..." but now you are right back into the one-step sparring problem from above.  If you're more creative you might come up with 1-2 more pretty solid applications for that move and maybe a handful of possible applications that stretch credibility a bit. 
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Want to know how to solve this problem and virtually all others when dissecting your kata into partner bunkai or one-steps? First step out of the way, then do the response from the kata. You might even be surprised to find things like low blocks turning into cool stuff like hammerfist attacks or cool armbars, punch chambers becoming blocks, holds, pulls, etc...
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Try this, choose any kata you like. Take it apart into its logical 1-2 step pieces and practice each one adding an explicit evasion (a sidestep or turn, etc...) to the beginning of each one (even if the evasion is not part of the kata). I think the kata will all of a sudden spring to life for you, and a whole bunch of the stuff that is weird and dysfunctional about karate will sort itself out very quickly.
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Try making the assumed explicit in your practice.