Performance Artist use chairs as material because they are cheep.
The other thing is that chairs are seen as representatives of “the man” a king of a sort. Not everyone has a chair to sit on and it is easy measure the prestige of a person if s/he has a seat at some event.
Also sitting is the easiest body position to mimic which has meaning. Standing does not look like anything. It’s neutral state but sitting implies that a person is idle or at work or both. A person sitting is in some state.
Chair are obstacles which superimpose the idea of “being idle” or “at school” or “at work”.
Images via http://www.eero.storijapan.net/aineisto/E_e/kuvakooste/