I couldn't agree more. In fact this was a major roadblock in my head when I first starting training. In many arts the uke simply does one thing, one grab, one punch, etc. Even in my 100% fresh noob brain at 12 years old, I couldn't understand this mentality. A person never just does one thing. They react, chain, move, adjust. They are living, breathing, intelligent, tacticful (not a word, I know), people.
My first TKD lesson was me learning a few basic techniques then some drills to practice them. I was told to throw a punch to the face, I threw it and retracted my arms and was told this was wrong. It didn't make sense then to stand there with an arm outstretched waiting to be 'blocked' and it doesn't make sense now. Beyond of course building the very very basic understanding of a technique or movement. I mean no one is just going to grab your hand, so why practice just grabbing your hand. Grab your hand to pull you in, or grab your hand to attack you with a strike, or grab your hand to enter in for some kind of body grip. Just like people never throw one punch and wait, they never grab and wait.
I always thought you didn't retract your hand because the other person needed to work on their blocks, it's about perfecting the block, not the punch. Later when they have the block nailed they can practice against propper punches. That's the way I've always seen things done anyway. "Nobody attacks like this" isn't always a valid response. In fact it's often the biggest bunch of BS floating around the martial arts at the moment.
People that have never boxed honestly think that if they attacked someone it would be as a boxer does, for instance and they expect everyone else to as well, even if they're untrained. But that's another argument.
The point of training is to develop skills and often the best way to learn those skills is often with attacks that may not be the most realistic, but are the most useful for developing confidence, timing, form, mai-ai ect.
I really don't care if no-one will ever attack me with shomen uchi, the practice of dealing with shomen uchi has taught me lots of stuff that is applicable across the board.
The point about grabs is that there shouldn't be time for something else. Where I train gakyu hamni katadori is a wrist grab followed quickly by a punch. We rarely do the punch because if uke can make the punch tori has already failed. Why would you allow someone to walk up to you and plant themselves ready to strike? That's madness, you don't win fights by giving someone a free hit or letting them take your balance. The defence against a grab assumes a follow up if not simultaneous strike and it begins before uke gets anywhere near making contact.
Sure, it sometimes looks like "grab and wait" but only if you misunderstand what's going on.