If we're going with anecdotes, I have learned Bash scripting and zsh scripting separately at different points in my life. Both times I forgot it very quickly. My guess is that you value that knowledge and that helps you retain it. Practice of course helps.
For me, my primary shell both at work and at home is xonsh, which is Python-based, and 90+% of all shell scripting I do is in Python in that shell. Having that knowledge of Bash is not even worth 2 days for me. If I were an embedded developer or a system administrator where I often have to SSH into accounts I don't control I could value Bash more but that's not the case for me and a fairly significant percentage of software engineers. Why spend a few days learning it when I don't need it? Even in the example above, I did it for my convenience, not because I needed it.
For me, my primary shell both at work and at home is xonsh, which is Python-based, and 90+% of all shell scripting I do is in Python in that shell. Having that knowledge of Bash is not even worth 2 days for me. If I were an embedded developer or a system administrator where I often have to SSH into accounts I don't control I could value Bash more but that's not the case for me and a fairly significant percentage of software engineers. Why spend a few days learning it when I don't need it? Even in the example above, I did it for my convenience, not because I needed it.