So, why should anyone expect China to?
In fact, if I was a Chinese government official I'd be laughing at anything the US suggests. Maybe I'd sign the pact just for a joke though.
the thing is, what the U.S. politicians - and many people around the world - don't realise is that the Chinese Intelligence is so secretive it doesn't even have a name. its members operate in effect as independent cells, through word of mouth contacts, with absolute negligeable two-way contact with the outside world... even inside china and *including with the politicians*. remember, china's politicians, under the "one party state", don't actually have much in the way of power, and are not really that well-respected (or trusted).
so the hilarious thing is that the only way for the politicians to inform the Chinese Intelligence that there's a treaty that's supposed to be signed is, in fact, to announce it in the news and hope like hell that someone relevant, somewhere, in their lair / bunker / hideout, actually reads it. here's the problem, though: if those operatives happen *not to agree* with that treaty, as far as "China National Security and Interests" is concerned, then, well, they don't actually have to take a blind bit of notice.
the same goes for when all these attacks keep occurring. the *simplest* thing to say is "it was chinese hackers! they're nothing to do with us politicians! we have a policy of not attacking foreign assets! no really!" because for the politicians to even *admit* that it was Chinese Intelligence operatives - not that they could possibly find out who they were even if they wanted to - would probably result in them getting a knock on the door and them and their family deported to some remote area of China which hasn't changed in several centuries.
we in the West assume that just because the Politicians in Western countries make the laws, that other countries have to follow that exact same process. China's politicians - people don't realise - are *not* at the top of the food chain as far as power is concerned. They're not even second to top. on mature reflection, you might call that a good thing, as it means that they can't really screw things up.