Training for gran paradiso

I have a spin bike at my home and I bought it exactly to train my legs and aerobic system for mountains. I also do bouldering, lead climbing. But all pro mountaineers are marathon athletes and they all are running instead of cycling. So my question is : is it really necessarily to use running ? At least in my case ? Is running that much better? Because i don’t really get why it can be that superior to cycling. It maybe more effective for ur aerobic system and etc but that’s pretty much it, like maybe 20% more effective? I heard people say it will help ur tendons but when I was hiking I didn’t feel any pain in my tendons or anything. Just my legs and breather. P.S im 18