https://github.com/sindresorhus/ama/issues/10#issuecomment-1...
TL;DR: Small modules are easy to reason about, and encourage code reuse and sharing across the entire community. This allows these small modules to get a tremendous amount of real world testing under all sorts of use cases, which can uncover many corner cases that an alternative naive inlined solution would never have covered (until it shows up as a bug in production). The entire community benefits from the collective testing and improvements made to these modules.
I also wanted to add that widespread use of these small modules over inlining everything makes the new module-level tree-shaking algorithms (that have been gaining traction since the advent of ES6 modules) much more effective in reducing overall code size, which is an important consideration in production web applications.