> hardware vendors optimizing Linux for their hardware This makes sense for component vendors that want their customers to use the hardware with their customer

> hardware vendors optimizing Linux for their hardware This makes sense for component vendors that want their customers to use the hardware with their customer's existing OS (Linux). If it's a hardware vendor that also controls the software (e.g. mobile phones, appliances), then this doesn't apply. Keeping modifications to themselves can serve maintaining competitive advantage, reducing liability, or simply reducing costs of publishing code (either getting it merged upstream or mirroring a fork).