I can’t speak to Bluetooth as I don’t know much about it from a software development perspective, but when I first started working with USB I was surprised to find that the hardware is really simple but there’s a ton of software that makes it work. Specifications evolve over time to add features or improve efficiency. This neccesitates code changes, and those code changes have to be made at both ends of the connection if you want the devices to continue to be able to talk to each other. Aside from maintaining the interface code, there are probably also changes and improvements to the device itself. Thus, firmware updates. Make sense?