Reciprocity is an ethical concept. I told you, there is no law that mandates people have to be nice to each other. Courts do consider matters of pure equity from time to time, but such disputes are heavily fact-based and don't generalize well. That's why contracts exist; they're a mechanism for establishing predictability between people who may have very different moral assumptions, but (ideally) negotiate agreements of limited scope to which they are both willing to adhere while still maintaining their different interests/ moral beliefs/ tribal membership etc.

Courts don't want to be moral arbiters if they can possibly avoid it because questions of morality are inherently political. Contractual relations are a mechanism for dealing with that problematic social reality.

Edit: if it helps, think of contract law as a language specification. The compiler cannot and does not evaluate the worthiness or value of a program, only its semantic consistency. An individual contract is like a program, the law as a whole is like the operating system, institutions are the firmware, physical assets are the hardware. It's an imperfect analogy but it might make the scope issues easier to understand.