It's the fitness function: Make a model which is capable of predicting the next token. The next token of what? EVERYTHING.

So what does this lead to? To a generic intelligence which is capable of responding/answering everything.

If overfitted, the model just remembers every possibility in the world but this is not possible anyway so it will start to identify patterns and rules and will use them instead.

Basically 'compressing' every possibility to every question someone could ask -> compression leads to intelligence.