It's not even just that amendments happen, its that as written laws the US constitution is terribly drafted. It is vague, doesn't define many of the terms it uses and is full of edge cases that it doesn't deal with.

It's more a statement of ideology than actual law, which means you need a whole system of court decisions that lay out the actual practical interpretations that courts are supposed to follow, which of course are decided based on the political needs of the day so they are a total mess.