Not in practice, because these UI libraries usually have many intertwined interdependencies. <Button /> probably still imports all of the context providers, which also import their own dependencies, etc. So you end up including half of the library if not more just for a button, but if you use multiple components from the library the cost is amortised.

And typically if you're just using a single component it's one that is more complex than a button, like a popover or colour picker. Which then pulls in other components, etc.