Latest example: at my current workplace we have some crappy custom compilers for our platform.... I've rebuilt them in a way that I like (and using C# instead of Java), that enables me to use it in a small visual studio code plugin that enables me with real time error messages as I code against our platform, so basically constantly compiling in the background as the file changes. It's great. Can it improve the lives of the 50 or so other devs working on the same platform? For sure (it saves a ton of time and frustration). Will I tell them about it or release it onto our private git repo's and make it part of our platform/build chains? Nope.
I can give you 20 good reason why I won't do it but really don't want to get into it. If they want it, they ask for it & while they are at it, they can pay me a cool sum of money for it. Until then, it is my own private competitive advantage that allows me to work 2 hours per day instead of 8. Not embellishing.