I think most do it simply because they either don't want to wait for the season to start (gun shots at dusk and dawn during bow season, my personal favorite), or they're not done hunting once the season is over.
Or if trespassing, they're simply too lazy to find a place to hunt legally.

Or if you ask a poacher he'll say something like; "Itts'a traydisyioun... ma grandady done it, then-ther ma daddy, now I figer itsa just in me, ya'no." or "man I just love seening those glowin lit'l I's, staring right at me with my million candle power spot lite. Tha tak ther theiving lit'l head outta the cern piyl and BLAM!" "Go sthyl sum body elsis dang cern,,, ha ha"