Here's a few ways of thinking about it:
1. Escalation, the instinctive response of angry humans, is generally a bad idea in international relations (or any relations). I once read a veteran practitioner express it this way: 'The job of international relations is to prevent a B-level problem from becoming an A-level one'. The U.S. and China, for example, have a very large number of interactions on an enormous number of issues; if someone retaliated every time they were unhappy - and then of course the other side would re-retaliate - that would be all that was happening, lots of treasure would be wasted, war would be risked, and positive things wouldn't get done. That doesn't mean that you do nothing, but you need to respond in a way that doesn't escalate the situation - it's not always easy to thread that needle, but that's what the professionals are there for. Another reason retaliation is bad is that it puts the other side in control of your actions - they can provoke you into doing what suits them, when it suits them. You want to act at the time and place and using the means that best suits you, not dance to their tune.
2. The U.S. does take action against them, but not in public. This helps prevent escalation - the Chinese government isn't publicly challenged and forced to respond.
3. Espionage is, in one sense, like a state of war. The U.S. is already doing all they can to shut down Chinese espionage and to spy on China, and vice versa. China isn't breaking any rules in the context of espionage (AFAIK), and if the U.S. could do more, they'd already be doing it.