If we wrote software to do this, but we were so incompetent that we couldn't fill in the model with correct data, we might just say "who gives a fuck, fill that with random garbage and we'll fix it in a later version". And then we never do.

Your subjective experience is that incompetent model. Your model doesn't know how to correctly judge human character, so you misread people and always wonder why they say one thing but another is true, and it doesn't click that they're lying the whole time. You can't keep track of time because the internal clock just isn't implemented, so the who day seems to drag on, or maybe fly by too quickly.

It's all just really shitty software. Layers upon layers. And because humans believe this to be some mystical thing, rather than trying to fix it from the inside, they assume that it's awesome, necessary, and why would anyone want to fix it?

No small fraction of it is simply because our memory is faulty. The only time you ever remember anything is the first time you remember it, every memory access of that is really you remembering the last time you remembered it. Each access is lossier than the last, and confabulation is guaranteed. This seems to be true even moments after the event.

If it was anyone other than evolution who wrote your code, you could sue them for criminal negligence.

And that's before we even get to the part where you find out you're not even you. Inside your skull is another being, an intelligent one, with desires and goals. But you can't see, hear, or feel this being. It's invisible. The "you" that I'm talking to, exists because this being once upon a time needed to simulate the other humans around him, so he could anticipate them well enough to not be out-competed. He has a pretty good idea what they'd say if he asked them questions, how they'd respond to threats and challenges, what sets them off (so he can avoid fights). And, by mistake or design, he used this simulator to simulate himself (maybe to bootstrap it? if the simulation's output matches his known answers, it's working correctly?).

You're the simulation. When the judge asks the psycho teenager why he put the cat in the microwave and he says "I dunno" he's telling the truth. He does not know why. When your girlfriend cheats on you, and she's crying hysterically and can't tell you why she did it, she's not just lying (either to hurt you or to spare feelings)... she doesn't know. It was that other being in their skulls doing these things. They're just the simulations.

Now, you've either been poking around in your own head, seeing little glimpses of what I'm talking about, making you wonder if I'm not on to something, or you're incapable of that. I've met both kinds of people. If you're the former, you're wondering just how much of it I understand, because some of the glimpses paint a slightly different picture from what I describe. That's because our minds weren't built the same way. No two are alike, not in a special snowflake way, but instead like no two shacks in shantytown have the same kind of leaky cardboard roofs. And, if you're the latter...