Not sure that's true. I did a 18 month stint with a tiny (6 person) hardware startup - most of my time was spent ensuring our devices could auto-update their embedded Linux securely without bricking or losing any of the custom hardware specific capabilities. It's not an easy trick to pull off (big thanks to the ARCH Linux team for all their work that I built on), and there's ongoing cost involved to ensure newly discovered vulnerabilities have a process to be evaluated, patched, tested, and deployed to the fleet of devices.