How is Vector3 implemented, why are the properties readonly?

How is Vector3 implemented, why are the properties readonly?

Vector3 v = new Vector3(1, 1, 1);
v.x = 5;
Why can't I do this? I have to do v = new Vector3(5, v.y, v.z);
I assume the reason behind this is for performance. But I can't guess at
why this is necessary.