The primary reason for excluding the node_modules directory when using babel-loader is performance optimization. The node_modules directory typically contains numerous third-party libraries, which are mostly precompiled JavaScript code and do not require further transformation by Babel. If babel-loader processes these already compiled files, it will significantly increase build time, thereby reducing build efficiency.
Additionally, processing files that do not require transformation may introduce unnecessary issues or errors, as the code in third-party libraries may have been optimized for specific environments or specifications, and retransforming them with Babel could break these optimizations.
Therefore, by excluding the node_modules directory, we can ensure that babel-loader only processes the application's source code, thereby improving build efficiency and reducing potential compilation issues.