Sugar itself isn't bad for you, TOO MUCH sugar is bad for you.

For like 99.99999% of our evolutionary history, food was scarce and unpredictable. It was practically impossible to find enough calories to eat too much. Therefore, having the trait "whenever you encounter an energy-rich food source, eat as much as you can and turn any excess energy consumed into fat" was a GOOD thing. Any animals that had this trait, including our ancestors, had a survival advantage! If you craved to eat as much high-energy food as you could find, you were more likely to survive a famine, drought, injury, or illness.

On the time scale that evolution creates changes, food becoming cheap and plentiful in the modern age happened extremely recently. It's only been like ~150 years that regular non-rich people could afford enough food that it's easily possible to eat too much. To evolution, that's essentially nothing. That's a single-digit number of generations! Human evolution takes tens of thousands of years or more to produce changes.

So that's the problem: We're all still running a program in our brains that says "eat as much sugar as you can find" because that strategy gave our ancestors a survival advantage for like a billion years, and now it's backfiring for the last hundred years or so and there hasn't been time to change the program yet.