Righteous fire is the most boring skill in the game

Righteous Fire is indeed boring, to look at, a big disc, not exciting at all. But it is quite interesting to build.

Sure, the game is much more enjoyable if you can accept losing and just play the game without copying guides etc.

just because 98% POE.ninja plays RF / ToxRain / whatever doesn't mean you have to it.

It's also why ruthless is good for the game, because you can't copy build guides.

it says they care about their wrist & joint health

If you're talking about softcore trade, people have never played skills based on them being good, they've played skills based on them being easy.

Righteous fire has the lowest skill floor in the entire game, so it's popular even in patches where it's garbage. It was the most popular skill last league, and it was objectively terrible for the sanctum money printing machine.

I use Righteous Fire but literally never for the AoE.

Righteous Fire gives 40% more spell damage without reservation cost.
Zealotry gives 15% more spell damage at 50% reservation cost.

If you can sustain it then it's the highest Spell Damage boost you can get. If anything you run Purify of fire at 35% reservation with +3% Max res and it's still better than most alternatives.

RF builds are good because they are nicely scalable and work with any budget, from leaguestart SSF all the way into minmaxed pog gear territory.

And it's not just RF, it's also Fire Trap. As you develop your RF character you will learn that RF AOE does enough damage even with a 4-link, it's bosses that are the main issue. So you transition into Fire Trap as the main 6-linked skill.

It synergizes well, as both skills deal fire damage over time.

There are better bossing builds, there are better mapping builds too. RF is a jack of all trades. Tanky, smooth, doesn't hurt your wrists.