为什么在线上很少有完整的数据库模式示例?

It is a bit strange to me about database schemas sample which I think most of the projects will need to design a database for it. but when I try to find sample online, it is really rare (eg. full database schema for Inventory Management, School Management, HumanResource management). I know that each database design is depends on the project requirement, but I don't think it is much different. It is a bit strange to me about database schema