Blocks are called blocks because they provide a good visual for the technique. But like many things in life, if you give it a name, it will be used according to that name. So, blocks became just that - blocks. And for many people, they do not use their brain to inquire further, trusting that because it's a block, that's all it's for.
Techniques in kata fall into two general categories. In Japanese, we call them "bunkai omote" (omote means "in front", and this phrase means "obvious bunkai"), and "bunkai ura" (ura means "behind", and this phrase means "hidden (some say 'secret') bunkai").
We in Karate and Taekwondo are often taught the bunkai omote because it is the easiest for us to understand, but a good instructor will tell us that there are many interpretations of a movement, and these are what are defined in the bunkai ura. In the bunkai ura, there is an entire world - right under our own noses, and we'd never be the wiser without a good instructor - that teaches us about throws, pins, locks, strikes, and a good many things other than what the technique is named.
Even our strikes are not always that: a clenched fist may be reminiscent of a punch; hands returning to the hip may be reminiscent of a chamber; and things like crane and tiger stances, spearhands, punches, knifehands, and other techniques have completely new meaning.
So in a sense, yes - blocks are unrealistic. Can you block? Of course. But in most cases, a technique named as a block does not work well as a true block. Perhaps in some bunkai ura, a block is a block if you can make the correct oyo; but in reality, such techniques are just placeholder (and given an unfortunate name) and it is up to us to find an application that fits the given scenario in the kata.
And often, even if you do come up with a good application as a block, you have explaining to do about what the other hand is doing, or what the stance is for, or why the movement is fast or slow.
Take, for example, a "double forearm block" (sometimes called an "assisted outside block"). What's the other hand really doing? Why might we be in a back stance in one case, but a forward stance in another? Clue: it's not even a block.
And what about a spearhand? This is famously expressed as a means to get inside the armpit or the throat. Or the solar plexus. Go ahead and give it a try, let us know how that's working for ya. In the meantime, accept it as a throw or a preparation for a throw or lock. Yes: it's no strike at all.
And what of the famous palm block? You know, the one which has you pushing down on an incoming punch, only to aim it at - guess what? - the groin?? Oof. Sacrifice the groin for the stomach and not even move out of the way. Got it. Or maybe it isn't blocking anything at all.
You will often find it more mind-opening to look for bunkai and reference the technique by movement (kinesthetics) rather than by the way it is spoken ("block"). That frees up the mind to understand more about what you're doing, so that you can account for the other hand, the stance, and the previous and next technique.