IoT devices can be all kinds of things, from vending machines, HVAC systems, industrial machinery, vehicles, and so on. Some of it might not be updateable over the air, and can be located in all kinds of inconvenient places. I wouldn't want to be the one that has to manually deploy an update to sewer flow monitoring devices, or heavy mining machinery that's in the middle of nowhere.

A very large amount of customers are not technically competent enough to successfully update the firmware over USB. Also, from the business side, normally you would do OTA updates for deployed devices; now you need to build and deploy a customer-facing USB update tool for them to run on their laptops. And provide tech support for it, etc.

For industrial or B2B IOT you would in many cases need to send out your own field techs to do the update, which costs $$$.

So overall, even if you're not literally doing a RMA, it is very very expensive.

some do but you need to actually take every device - which you may have sold to customers - and update. Industrial stuff has multi decade contracts often in remote places.