Why are there not loads of skyscrapers near the subway stations?

My understanding is circular subways are the most efficient (people can travel / commute in both ways fully utilising the infrastructure). I was astonished to learn Glasgow has exactly one of those (and was about the third subway in the world!). On the face of it Glasgow is blessed with incredible public transport infrastructure. But please can someone explain why there are not tonnes of high rise condominiums / office blocks surrounding 5 minutes walk from them? Just seems such a waste of a resource and Glasgow seems so low density and spread out - meaning very few people actually live near the stations??