Nobody alive observed the rate of the growth of them a hundred years ago. But the size of them was noted a hundred years ago and is being observed now. A thinking person can assume that the rate of growth 50 years ago, 100 years ago, 250 years ago, 700 years ago, 2000 years ago, 5000 years ago is the same.History cannot be observed again. A scientific theory has to be observed before I take it seriously. Many aspects of evolutionary theory have or are being observed, so I accept them. No one observed the rate of growth of a stalactite.
Did the caves they grow in appear suddenly or did erosion create them?