Every social medium tries to keep people on their platform.
It’s their business model. They try maximize the time and attention you spent there.
Moreover people are there because they like the platform.
They are used to it and it is comfortable for them.
Facebook does not promote your posting of YouTube videos.
They want people to stay on Facebook. Even people on Facebook prefer to stay on Facebook!
You can ignore this and fight this,
or you can choose to serve people with content on the platform where they are.