TL;DR because they have to. I like this quote “The reality is, everybody is hacking everybody, and have been doing so for a long time. From North Korea to Russia to Vietnam to the USA and even Australia, they are all spying and being spied upon.”…:
[…] No-one has handled it very well and essentially the government is now throwing more money at the problem. Defence have won the battle, though, and are now setting the agenda for the future of cyber security. With the rise of foreign interference threats being seen in higher education, politics and business, our new focus is clearly more about national security and defence than law enforcement: it is to attack those who threaten to bring us down by cyber means.
China is a major concern but the threat actors also include Iran, Russia and others. Whatever conflict you see in the real world is also happening in the cyber world. If we are having bad relations with China in the real world, you can bet the cyber world is having the same thing.
There are a lot of things that a foreign power can do with an offensive cyber capability. It is quicker to set up and cheaper than most other forms of warfare – think about a military aircraft or missile bombing of a uranium enrichment facility versus a cyber attack such as with Stuxnet. And you’ve got the ability to be more or less anonymous. It’s also deniable.