There's a phrase in the outdoors community, especially when hiking in the mountains.
"Cotton kills"
When cotton gets wet it doesn't insulate as well as wool or synthetic material and it takes a long time to dry.
While this is true, you can definitely hike in jeans without an issue if the weather is good and everything goes according to plan which realistically is like 99.9 percent of hikes.
It's probably easy for the club to have a rule against wearing jeans so they don't have to worry about the weather turning bad one time and someone getting hypothermia.
Personally I hike in shorts or dedicated synthetic hiking pants because they're more comfortable. They cover a wider range of conditions and temperatures as well as being easier to move around in. Jeans just aren't designed with hiking in mind.
That said, do what makes you happy, I'm just some dude in the internet.