Because they are tricking you into a back door debtors exam. Once they know your assets and if you have any, the negotiations stop and they press ahead full speed with the arbitration and eventually court case. As I said earlier, this is an OC. An aggressive OC who will usually have everything needed to win a case. Unless you have some leverage somewhere or can use the BK card as a nuclear option, they do not back down.
If you had offered 90%, they may have taken that just to save everyone the trouble. But since they asked, you tried and at least now you know where things stand. FYI, I am helping a relative in an arb case with them also. But we have the BK option on the back end along with fully exempt income from garnishment. So I have a few cards to play.
Do the best you can for yourself, but it will be a tough road.