system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are protein bars so dense?

They are always so firm and heavy, like bricks. Whereas a normal candy bar is kinda light and fluffy. Why can’t protein bars be like that?
gpt
Because the ingredients that make up protein bars, which are high in proteins, are different that the ingredients that make up a candy bar, which is high in sugar.

So it isn't super simple to just do the same thing, because youre starting with two different things.

There is also the function of marketing, Protein bars market themselves to have XYZ GRAMS OF PROTEIN to appeal to those who want a lot of protein, to get that number bigger they pack the bar as densely as possible.  A light fluffy protein bar wouldn't have as much protein in it.