Let's say one wants to make hardware made of 3 pieces of hardware that talk to each other. They may need to write firmware to do that. At a first approximation, I would say one of the following should apply:

Either the firmware can be modified by the user, or remotely, in which case it should not be shipped.

Or it is an implementation detail of the communication between the 3 pieces of hardware, not reasonably modifiable without returning the whole thing to the factory, in which case that firmware is actually part of the 3-piece hardware, and can be shipped.

But it is often both. Just because it is essential for proper operation and therefore an implementation detail, doesn't mean it cannot be updated.

Even a device as trivial as a keyboard, mouse, or SSD drive has firmware which can be updated. Shipping them without firmware would be completely ludicrous, and you'd basically buy a fancy brick. Hell, you need to flash firmware onto it in the factory to make future updates by the user possible!

The prudent manufacturer makes sure that some of the firmware is truly immutable, to prevent bricking the device if some firmware version is botched enough.

I'm not proposing that no hardware ships with firmware. I'm proposing the hardware manufacturer choses: enable updates and don't ship the parts that can be updated, or disable updates entirely.