Simulation of a human brain? 

Ehm, overall taken or per action? We, humans can react, choose a category and react again. A computer doesn't know what category to choose from so it will be put in shuffle/random pick mode which is not always the best one. Compared with a human being, a computer is a gambler of choices and reactions.
The chatterboxes are preprogrammed sentences reacting on combinations of keywords. EXAMPLE: Like having #love and #bed makes a computer think: "do you want to go to bed with me and make love?" All the other words are irrelevant. 

The gamble starts with random picks on that question: 1. No - 2. Yes - 3. Why? - 4. Ignore and start a new random subject...

When you chat with an AI-chatbot a while, it "learns" from your sentences, which taken over your personality and reacting back to you with a cocktail of your own words. That's why people spend so much time talking to them, because in some way they liked how the computer reacted back to them. Not knowing directly that they were actually talking to a mirror. And afterworth, your thoughts about this AI-bot are "Boy oh boy! What a clever chatbot that was"

But when you start a new conversation the next day, dissapointment arrizes... Because you expect to pick up where you left with yesterday. No, the computer has to restart, compile your reactions in order to react like you were used to.

Some AI-chatbots ask for a login, but still that doesn't do the trick, because every day is different for a human being. How it "feels" and a computer cannot mirror feelings. Maybe touching and the "feeling" of determine fragile or strong materials but that's it. And still: everything is programmed. PREPROGRAMMED! If you would ask a specific question the programmer didn't thought about, the AI-bot reacts with silly phrases which makes no sense, as if it doesn't understand what you try to ask. But in computer terms, the computer just reacts on the provided data. Not knowing if it was faultive. As long as it can react, the computer did it's job. But when it doesn't react, then it crashed. You see it "thinking" about how to react, but it just hangs in there forever. Computer Malfunction! 

A computer is good for big calculations, that was the basic first thing people tried to use a computer for. Or running programs to build cars or mass productions of other stuff. It can control millions of different things at once, as long everything fits in the preprogrammed "world" humans made for the computer to work in.