My experience of festivals is that there is either a supplied (good) backline that you are required to use as a condition of playing, or that everything is on movable platforms that can be wheeled off to a backstage area for setting up and packing down.
I think the sorts of gigs people are talking about here are those in sub 500 capacity venues where there is often no backstage area and the green room is already full of all the other bands and their kit. My attitude at these gigs is, the moment we have finished playing, to get our gear off stage as quickly as possible, find a quiet corner and then pack it down. However, some musicians still don't appear to have got the message and can take up the whole 15 minutes on stage packing their guitars and pedals away whilst chatting with their fans.