I really hate that expression. Nobody EVER reinvents the wheel. We all know about wheels, and how they work and why they are useful. But we invent new types of wheels all the time. Maybe we put teeth on it and invent the cog. Maybe we invent the tire to improve traction and reduce bumps. Maybe we put spokes between the hub and the rim and to reduce rotating weight. These are all new and useful inventions despite still being wheels.
Sometimes an existing wheel design is perfect for our use case, or at least good enough that creating a new one isn't worth the effort. Sometimes a tweaked wheel is what we need. Sometimes a radical departure from established wheel design is required to make a new machine possible.
Build vs buy is an eternal question, and it's reasonable to examine and reexamine it for any project. But "reinvent the wheel" is just hyperbole.
People absolutely build solutions to problems that are already solved, often worse than the existing solutions.
Your complaint about the phrase seems to be that it doesn't well describe the process of adjusting another design to work better for a specific use-case. But that's not what the phrase is trying to describe, it's literally describing the problem where people reinvent that which absolutely does not need to be reinvented.
An average engineer in any field probably has developed enough systems thinking to reason a great deal about the variation of wheels across applications.
I've seen many applications re-created feature for feature just to do it in a different language. If it doesn't improve the wheel or solve a new problem, that's reinventing the wheel.
My point was not that nobody ever does unnecessary work. That does happen, obviously, and we can certainly object to it.
In the case of rewriting an application feature for feature in another language, that may or may not be a good idea. Maybe we're switching from an interpreted language to a compiled language because we need better performance. Maybe we're replacing a huge pile of impenetrable bash with a well organized Python app. Maybe we're taking advantage of a library that's only available in another language. Maybe there's a good reason for the rewrite but it's so much work that it's still not worth it. Maybe it's just a engineers wanting to use the latest toys because it'll look good on their resumes.
It's totally legit to question the need for a given project. But the phrase "reinvent the wheel" is just scornful insult. It adds nothing to the discussion, and in fact makes it harder to discuss trade-offs.
Sometimes an existing wheel design is perfect for our use case, or at least good enough that creating a new one isn't worth the effort. Sometimes a tweaked wheel is what we need. Sometimes a radical departure from established wheel design is required to make a new machine possible.
Build vs buy is an eternal question, and it's reasonable to examine and reexamine it for any project. But "reinvent the wheel" is just hyperbole.