Why TV stations never have any downtime?

I worked on broadcast integration for years. With any serious broadcaster there is plenty of redundancy built-in and every equipment (that can be) is synchronized to a common timing source (black burst back then, tri-level and PTP today).

Since most (usually all) signals are easily routable any distruptions can be kept to a minimum. Upgrades and updates are always taking place, you won't notice when Master Control is upgrading it's playout system, since they route to the signal from the backup system (without a glitch since they are synced) while they perform the upgrade and testing.

Upgrades to equipment that isn't in 24/7 use is easier. You can upgrade equipment for a studio when it's not in use or the programs that usually use it will move temporarily to another studio or do outside broadcasts.

The incremental cost of running the transmitter is low enough to justify keeping it on all night. There are always people who can't sleep or are working graveyard shift and they will tune in.