Question

Why are indifference curves parallel?

Answer 1

Indifference curves are not parallel (except maybe in rare circumstances). Their defining feature is that they don't intersect.

Answer 2

The further an indifference curve is from the origin, the greater the utility provided along that indifference curve. If two indifference curves were to intersect at any point, it would mean that the same combination of goods was providing two different amounts of utility. Since this is impossible, indifference curves, by definition, cannot intersect.