> it's more of the fact the hardware is locked and serialized, e.g. it won't accept a blue tooth module unless it's unlocked with proprietary software.

Manufacturers have a regulatory obligation to prevent unauthorized combinations of software and hardware where those combinations could compromise the device's compliance. This is particularly relevant to radio firmware, including Bluetooth, where the manufacturer must ensure that the radio equipment remains compliant with the applicable regulatory requirements.

> The hardware manufacturers spend quite the effort

Because they are required to do so by regulation. Those regulations exist for good reason, for example to prevent unwanted interference in the radio spectrum.

Beyond regulatory requirements, manufacturers also have legitimate reasons for preventing unauthorized tampering with their devices. One concern is theft and device resale, while another is software security.

You may argue that you don't care about these things, which is perfectly fine. Everyone is entitled to their own opinion, and there is nothing inherently wrong with holding it. However, I would argue that this is not what the vast majority of customers expect, and that most would ultimately accept these constraints for the benefits they provide.