Why does the value of sin30 not change if the triangle in which the 30 degrees is found is enlarged

Because it's still a 30-degree angle, no matter what size the triangle is.

If you think of the sine as the ratio of the opposite side divided by the hypotenuse, remember that as you enlarge the triangle, both of those values increase by the same factor. Their ratio will remain constant.

The sine depends only on the angle, not on the scale factor you apply to the sides.