It’s because of a time and space-saving shortcut that’s used in the final stage of the manufacturing process.
Aluminum, like all metals, is malleable; that is, it will squish when enough pressure is applied. That’s in distinction to most other solid materials, which will crack under pressure. So metals can be rolled out into extremely thin sheets.
Metals are malleable because their atoms are held together by a moveable sea of commonly owned electrons, rather than by rigid bonding forces between the electrons of one atom and the electrons of the next, as is the case in most other solids. In effect, then, it doesn’t matter much where a metal’s atoms are with respect to one another, and they are therefore free to be pushed around within the electron sea.
In the aluminum foil factory they roll sheets of aluminum through pairs of steel rollers that get progressively closer together, which squeezes the aluminum down to progressively thinner sheets. Household aluminum foil is less than a thousandth of an inch (two-hundredths of a millimeter) thick.
To save space in the final rolling, they feed a sandwich of two sheets at a time through the rollers. The top and bottom surfaces are in contact with the polished steel rollers and come out nice and shiny. But the inner surfaces of the sandwich are pressed against each other, aluminum against aluminum. Because aluminum is so much softer than steel, these surfaces press into each other somewhat, leaving a rougher, duller surface when they’re separated. It makes no difference whatsoever in how you’re able to use the foil.
And by the way: I hope you’re not one of those people who sometimes call it “tinfoil.” A foil is a very thin sheet of metal, any metal. Aluminum foil is a thin sheet of (surprise!) aluminum metal and tinfoil is a thin sheet of an entirely different metal: tin. Tin is a rather heavy, nontoxic metal whose foils were used as food and medicine wrappers before aluminum became cheap and widely available. But habits die hard, and many people still call aluminum foil tinfoil.
Someone should also let it be known that “tin cans” aren’t tin, either. A “tin can” used to be a steel can lined with relatively noncorroding tin on the inside. But these days the linings of steel and aluminum cans aren’t even tin; they’re plastic or enameled coatings.