A few reasons: - PureComponent was only added relatively recently. - While shallow equality comparisons are relatively fast compared to deep equality comparis

A few reasons: - PureComponent was only added relatively recently. - While shallow equality comparisons are relatively fast compared to deep equality comparisons, they still burn CPU. There's also a number of reasons why props could have changed, such as object literals being declared in the parent's `render` method, or the component receiving other components via `props.children`. In those cases, the "pure" comparison will fail every time, and be useless. - There is no "default" per se. Application components explicitly extend a React component base class - either React.Component, or React.PureComponent.