There are a multitude of design systems that exist today, that is true. However, the reasoning behind each of their existence depends on context.
There are many that exist as simple boiler plates to start from. Examples that come to mind are. These can range from simple UI kits, to a bit more advanced "design systems" (my reasoning for quotes will be addressed below).
Often, these exist to be customized and give you an easier starting point. They may also teach less experienced design tool users best practices when it comes to building a design system (how to best use symbols, libraries, components, etc.).
Then there are design systems tied a company or larger project. Some of these may be open source, some may not. They may be leveraged by other companies, teams and users to improve their own design systems.
The difference between these two examples is important. It's the reason I used quotes above in my response. A design system should be created in the context of your organization, product and users. It should explain how to use it, why you made the decisions you made and be much more than a simple UI Kit. That is what makes it most valuable to those leveraging the system within a specific product. When conventions and guidelines exist, teams can more easily focus on other aspects of problem solving.
Because of this, no two design systems will be the same. Sure, they will share similarities, but because each product will encompass different users needs and business needs, the systems as a whole can be dramatically different. This is why most design systems are built from scratch...or use some sort of starter template and then are heavily altered.