In principle, you want the output of a build to always be the same if the inputs are the same. In terms of correctness, you want "sameness" to be defined in terms of the contents of the input, not their timestamp, since timestamps can be easily be changed inadvertently by things like `touch`, 3rd party tools, etc. Also, relying on timestamps could pose problems for caching/checksums/etc if they are printed into any transitive dependency of a build pipeline.
Furthermore, timestamp precision is still problematic. Some filesystems still have one second granularity, but even if you use a filesystems capable of storing sub-millisecond timrstamps, that doesn't mean commands that get invoked as part of the build process (e.g. cp -p) invoke the right APIs that make use of such precision. e.g. see the doc for the .LOW_RESOLUTION_TIME directive of GNU make
