One reason is that disparate impls of POSIX utils don't behave as similarly as one would hope. It's easy to cross the line from using flags that are guaranteed to be available to using flags and behaviors only available on your machine.

An example of a mistake I've made like this is relying on `find` on linux to default to searching the pwd if it doesn't have a positional arg. Another is `sed -i` requires a file extension on MacOS, but doesn't on Linux. You can catch these with tests to some extent, but how many people bother running their CI on MacOS just to avoid breaking other devs?

That said, I did work on a production system which happily shelled out to `dig` for DNS lookups for several years; everybody who saw it had a visceral negative reaction, but it also worked well enough that there was no good reason to spend time replacing it.