When I used to work for a large mobile network, we would spend ages testing the firmware of new devices. Even giants (at the time) like Nokia would release phones which couldn't dial the emergency services, or put out more than the legal limit of radiation, or were in other ways defective.
We'd test, send a report back, wait a few weeks, get a new firmware, test again, repeat until everything worked. It took months. That was fine when phones weren't expected to be updated by users.
But when more modern phones arrived with flashable firmware, customers couldn't stand the delays associated with testing. They'd see a new firmware had been released and complained that the mobile network operators were delaying progress, dragging our feet, deliberately depriving customers of something cool.
The fact was, operators very often didn't certify the firmware because it contained *dangerous* bugs. I'm sure there was also a cost element - why pay to re-test a phone that you're no longer selling? - but that wasn't the primary driver.
Well, the manufacturers and customers "won". If you buy a phone through your network, it probably has a network-certified firmware blob. If not, you're at the mercy of the manufacturer.