Elevated baby playground

Why do parents chose to play with children down at their level, rather than raising them up to a suitable height? Probably the answer to this question is safety. But anyway, why not build a set of raised platforms that babies/toddlers can crawl around, with guard rails to stop the babies falling off the sides. The platforms could be arranged into a ring, with a lifting bridge allowing adults to enter the centre of the ring. The base of platforms could be used for storage, making efficient use of space.