The other issue is size of update.
There are few set top boxes or TVs without some connectivity now. Even the most basic have USB. Those that don’t have been around for years and don’t need an update.
Those TVs and boxes that are on sale now have update sizes in the multi-megabytes which would take a long time to download over the engineering channel anyway. Those engineering channel updates are costly as well, certainly £000s if you need the whole bandwidth for a big update.
It just makes sense to deliver updates in a quicker, more convenient form.