In software engineering, we call that Not Invented Here syndrome - rather than reuse a perfectly good piece of code (or game mechanic, or whatever), the engineer reimplements it. And so, a lot of time and effort is spent reinventing the wheel, time that could be spent refining and improving the original.
You're not wrong about NIHS being a problem, but I don't think that applies here.
Someone suffering from NIHS doesn't just reimplement it, they make a different solution to the problem, rather than use an existing solution.
Making yet another dice resolution mechanic, skill system, combat system is NIHS.
A designer wants all the parts to be seemlesss. Phrasing the game design as a Frankenstein's Monster implies a clunky implementation of cobbling parts that weren't meant to go to gether. It's rather insulting.
A designer might use ideas from other sources, but to the designer, the effort of making them not just fit together, but to actually become one system.
In software, it's the difference between integrating systems and creating a piece of software that encompasses the features that are sold in seperate projects. Getting stuff to work together can be a challenge, but it's not in the same calibre of effort or complexity as actually making a larger piece of software.
As to why this may or may not be a bad thing, if you can see the seams, it's a Frankenstein's Monster of a solution. Designing from scratch tends to avoid that problem, but takes a lot more work and risk.