Given other tools are available is loading the question. Right now almost all Unix-like systems have Python, but which version 2.7 or 3.x? We still have boxes in our environment that only have 2.6. If they have 3.x, what is the x? Script portability in a diverse environment is a crucial issue for system scripts and even a lot of application support scripts.

Then there's flexibility. With bash you have a massive ecosystem of command line tools, including the ability to in-line code for tools like sed, awk and perl right there in your script. Python has a great standard library, but often the best way to do something in Python is using a third party module, so now you need to curate a module library across all your boxes. Also suppose your script needs to do things other than just process text, suppose I want to ssh onto a box and run a command? Is Paramiko installed, no? So now I need to invoke ssh in a sub-shell which is a PITA in Python compared to bash. Yes it's easier in later 3.x versions but now we're back to version soup again.

We have bash scripts running in our environment that are probably 20 years old, and if I write a new one now it will still work fine in 20 years time. Often it will be a quarter of the length of an equivalent Python script and much easier to understand and reason about.

Then again, there are many cases where a Python script makes a huge amount more sense, often run as part of a bash script.