Climbing spots are usually in very remote places. No matter how many people use public transport there will never be a bus that goes 5 km up a mountain on a dirt roads to a place visited by 10 people a day during weekends. With hiking it’s easier because hiking trails start in many different places, often fairly well connected but many outdoors activities like climbing, paragliding, cannoning, speleology or even mountain biking only happen in very specific, remote places. I you have to hike for 4 hours to get to those places those activities become way less accessible.