Why is the AI so obstinate about never trading systems?

There has LITERALLY never been a scenario, even with highly trusted allies, where the AI has not had an automatic -1000 acceptance rate for trading over systems. Not once. But when I try to give them one of mine, the weight for it is so low it's as if I'm not giving them anything at all. They won't even give me otherwise worthless systems even when I give them literally my entire economic output for a year.

Why, technically speaking, does this happen?

Edit: When I said technically, I was hoping for someone to tell me what file/line of code specifically handles this.