Eh thats not always the case, imagine this scenario. You are on an integrated course, your agreement states that you solo after X number of hours and that you will take your skill test after X hours. Both are minimum numbers, so if you go over you will incur a charge. Its simple economic sense.
When I did my PPL my instructor hopped out of the plane at Immokolee and let me do my touch and goes, I did 5 approaches, 4 T and G's one Go Around and I was in radio contact with him all the time.