Why does BDSM involve a contract agreement?
It doesn't. Before you ask why, ask if. Contracts feature prominently in certain genres of BDSM fiction aimed at an audience that does not participate in BDSM, but in over twenty-five years as both a BDSM participant and educator, I think I've met perhaps two people who actually draw up written contracts. I know I never have. A tiny handful of folks who practice BDSM like the idea of a contract; it's part of the fantasy for them. But it's a pretty niche taste.