Maybe they researched and found actually all the options out there have whatever drawbacks that they don't fit the bill. Maybe they didn't bother to research because it only took them 10 minutes to script that manual process we perform multiple times a week, and they were curious to see if they could improve that. Maybe some of the 5 were pretty good actually, but the coworker has some specific ideas how their "6th" new tool could be better in certain important ways.
The good off the shelf tools make it easy to build on them with custom tools to fit your purpose.
I really don't mind if people lean a bit one way or the other in their inclination toward custom tools. But I do expect people to be supportive of attempts to build nice tooling, and show a curiosity toward the trade offs, and willingness to try an experiment.
Don't be afraid of building a tool, hacker friends. If nobody else displays an interest in what you build, or it doesn't turn out to really save time, or it costs too much to maintain.. you can ditch it! You are a professional with good critical judgment whose ability to improve the productivity of your group and provide repeatable "executable documentation" will carry you far in your career. Let your creativity shine, and have fun.