Because sin and cos are about ratios, not pure side lengths. For example, 2/1, 4/2, 6/3 are all equal to 2 even though we are dealing with different numbers.

Sin is the same as opposite/hypotenuse and cos is the same as adjacent/hypotenuse.

So the equation is basically:

(opposite/hypotenuse)^(2) + (adjacent/hypotenuse)^(2) = 1

Even if the sides are different, the constraints of a right angled triangle (which is what it is based on, not the unit circle) make all the other sides change such that it still equals 1. Rearranging the equation we get:

opposite^(2)/hypotenuse^(2) + adjacent^(2)/hypotenuse^(2) = 1

opposite^(2) + adjacent^(2) = hypotenuse^(2)

Which is simply the pythagorean theorem.