Why do supports do this?

Recently I have been playing AD, and whenever I do, I noitice almost EVERY support just stand behind me in lane. Alistar, lulu, blitz, all of them. Now, I am not bronze as you may think, I am gold 2. Ill just give you an example of my last game. I was playing vayne with a lulu support against a caitlyn and thresh. I was being pushed in hard, farming under turret all the lane and the lulu would sit behind me while i was under tower and just shield me every once in a while. Meanwhile, the other bot lane is free to poke me out and deny me farm. This isn't the only game either. Do supports just think the ADC can farm safely? I mean as support, you should be in front of your AD (in most scenarios) and trying to at least show some sort of presence. Is this just me or do most supports just think it is OK to let the adc farm while getting poked by the adc and other support? I don't know too much about ADC but whenever I am stuck in it I am either with a support who just sits behind me or against one.