It seems that we only practice "Psach Picha l'Ilem" when it is likely that the defendant would have said this himself if he would have known how to speak up properly. In the Gemara Gitin 37b most people do make a Pruzbul. This is because since Chazal made a Takanah that you should do so, therefore we assume that people do. It follows that even if somebody does not say that he possessed a Pruzbul, we can ask him whether he actually did have one and then it got lost, because this is the most likely thing that happened since most people possessed a Pruzbul to start off with.

In contrast, in our Gemara, there is no special reason to believe that the squatter at one time possessed a Shtar for buying the field. It is just as likely that he is living in the field even though he never bought it, as it is that he bought it with a Shtar, and then lost the Shtar. Therefore we do not invent for him an argument that he did have a Shhtar if it is likely that he did not.

Chodesh Tov

Dovid Bloom

It seems that we only practice "Psach Picha l'Ilem" when it is likely that the defendant would have said this himself if he would have known how to speak up properly. In the Gemara Gitin 37b most people do make a Pruzbul. This is because since Chazal made a Takanah that you should do so, therefore we assume that people do. It follows that even if somebody does not say that he possessed a Pruzbul, we can ask him whether he actually did have one and then it got lost, because this is the most likely thing that happened since most people possessed a Pruzbul to start off with.

In contrast, in our Gemara, there is no special reason to believe that the squatter at one time possessed a Shtar for buying the field. It is just as likely that he is living in the field even though he never bought it, as it is that he bought it with a Shtar, and then lost the Shtar. Therefore we do not invent for him an argument that he did have a Shhtar if it is likely that he did not.

I found that the Yad Ramah (#113) in our Sugya addresses himself exactly to your question. He writes that we do not invent arguments for people in monetary matters with the exception of Pruzbul. The reason that Pruzbul is different because nowadays it is only mid'Rabanan that the Shemitah year dissolves all debts (see Gitin bottom 36a that this is the opinion of Rebbi). Therefore since mid'Oraisa the lender can reclaim his debt even without a Pruzbul, it follows that Chazal were lenient and permitted suggesting an argument to the lender because it is only mid'Rabanan that he requires a Pruzbul. The opinion of the Yad Ramah therefore is that we only suggests arguments for d'Rabanan questions, not for d'Oraisa.

Here is an additional reason that we ask the person if he wrote a Pruzbol. This is because one can write a Pruzbol without the knowledge of the borrower, so therefore we say there is no reason to assume that the lender did not write one. In contrast one can only write a Shtar on the sale of the field with the cooperation of the seller, so on the contrary we have no reason to believe that he wrote one.

KOL TUV

Dovid Bloom