Because Strawinsky stresses the active and critical element of carrying on an activity. The comparison with real estate is in fact a good one: one cannot maintain it without replacing worn-out elements with new and different ones. But is improvised music a tradition in this sense? Well, at least a collective endeavour, isn't it? And in a wider sense, I think we get inspired by and react against our ancestors and influences, even if we do not at all imitate them.