I agree with you for the most part that there’s no reason to connect them to the internet, however:
Most modern TVs have Bluetooth and WiFi radios, therefore they’re never truly isolated, and consequently that means if there’s a security flaw, it can potentially be exploited without physical access.
Now your priorities (and frankly, hardware) will obviously differ from mine, but that risk alone is enough of a reason for me to connect things up in order to receive software updates. Of course, the privilege of getting software updates for your telly is not ubiquitous, but most manufacturers can issue updates if there is a good enough reason to do so