Because if you pay someone using a JM tx (which is the goal when acting as a taker) the receiver can easily know your change output, which is not good.

JM provides good forward privacy but in some case really bad backward privacy IMO
And if a taker gains no backward privacy, why would he even use the tool to spend in the first place ? Which lead to incentive problem because if there is no taker there is no Coinjoin anymore.