I could only figure out what you meant there because of your splitted question in portuguese.
You ask for the title, again. Good catch.
It's a question without an answer. A simple one at least!
Almost all of our information technology (what people call tech) usually is far from simple and there is no single reason for it.
Maybe we could call it stupidity, but humans are not that stupid. We're more like lost in the middle.
We could call it too much information. The world have about 8 to 9 billion people. That's a fucking lot more than our weak brain can process. We can only really humanize about 150 people. So in here I'm also blaming the brain capacity, but that's what computers are made for - to help minimize that impact.
We could even call it a social issue in which we're not really allowed or encouraged to use tech. It's too nerdy, don't do that, our brain is capable enough and "we only use 10%" right? Not really.
Why is it so difficult? Because it is. Maybe a better title would be "It is so difficult for a technology to be simple. Why, oh why?!". Like an airplane crashing, there are just too many variables to choose only 1 to blame. We must not.