Hiking

I think this is going to depend a lot on the sort of environment you're hiking in

For me, one of the big issues with jeans is that they don't dry out easily, if there's any chance you're going to get wet or sweat a lot, they're a very bad choice. In some cases I'd even consider it to be a valid safety issue if for example the temperatures are going to drop and damp clothes are going to put you at risk of hypothermia

Or in hotter, humid climates, they won't breathe well, meaning your sweat won't be able to evaporate which is how your body keeps cool (already enough of an issue when the humidity is high) on top of jeans already being kind of hot on their own

Also chafing, potentially fungal issues, etc.

And if you're doing sort of a more technical hike where you really need your full range of movement to climb over things, jeans may be a little stiff for that (although arguably a worthwhile tradeoff for them being more abrasion-resistant)

And if you're doing overnight backpacking, they're absolutely too heavy to be worth it in my opinion.

But I can think of plenty of hikes I've done where jeans would have been an adequate, maybe even preferred sort of pants, like if it's a day hike, the weather is cooler, dry, and maybe somewhere with a lot of rocks and thorns.

This kind of feels like a rule that was instituted because too many inexperienced hikers showed up in jeans when they were a very bad choice for the conditions and had a bad time because of it, so instead of trying to judge it on a case-by-case basis weighing experience levels, everyone's personal comfort, the weather and trail conditions, etc. for each hike, it was a lot easier to just say "no jeans" so that everyone could just show up and hike instead of having to play wardrobe police every time they met up (and maybe to hedge their bets against getting sued if someone ended up with hypothermia or heat stroke or whatever from wearing jeans when "no one warned them not to")