your question is not simple
requires at least a familiarity with learning Gemorrah
but those that hold it is stealing it is because the party losing did not expect to lose and does not willingly give up his money to the other
you will certainly say, of course he knew he might lose and of course he gives up the money willingly, he gives it up doesnt he, he doesnt protest or put up a fight. i cant explain it further but theres your answer, ask a Talmid Chochum to explain it better.