There are some cases where it may make sense to write your own implementation, if you have a niche microarchitecture that has unusual performance characteristic

There are some cases where it may make sense to write your own implementation, if you have a niche microarchitecture that has unusual performance characteristics that the OS doesn't provide optimized routines for by default. But for most u-archs the default optimized routines should do a good job. Things like malloc are quite a bit more complicated, and more workload dependent so there's still some opportunity specializing an implementation there.