Simple: Copyright isn't always enough to protect your IP when you are a small company. They do want allow repair-ability, they don't want to make unauthorized clones too simple.

Bigger tech companies don't have to worry to much about this (given how they operate, legal teams they have, influence they have etc.). For small companies in the bootstrapping phase it's a problem.

Also there might be legal requirements, like some small parts of the layout being based on Intel provided schemas or similar. But that last point is pure speculative and irrelevant as the first point is good enough of an reason (while your company is in the bootstrapping phase).