One reason: let's say you write tool 'foo.sh' in Bash. It does what you want and you move on to other things.

It's suddenly a year from now, and your foo.sh tool needs some new features, or is too slow to do the job any more because your requirements have changed.

You decide it's grown too much for Bash and want to move to Python for maintainability, or Go for performance, or C++ to link with some library you need to use.

Now you have to tell your team (and any other teams that have found your tool useful): "We only want to maintain one version, so don't use 'foo.sh' any more. You have to use 'foo.py' (or 'foo.exe' or whatever). Oh, and have fun changing all YOUR scripts and tools that reference 'foo.sh'!"

That's one reason.