I suppose because you want to have more ships to store your nuclear armed planes in more places around the world. Speed.
Honestly (and unfortunately) if anyone actually did destroy a US aircraft carrier, the counter strike would be immediate and an order of magnitude more devastating.
Why? ICBMs are good enough and much cheaper. Also, launching missiles from submarines can be very effective.
The thing is, not many things in the world cost more than an aircraft carrier. What's you're going to blow up? Also, how, short of strategic nuclear war.
China is playing the slow game and they will continue to play it for a long time to come. But of course the slow game also has lots of issues for them as well. Issues with their population, with India, with the regime and so on.
China is in a position not that different from Germany pre-WW1, but arguably even better. But unlike Germany they are not controlled by complete war glorifying idiots. China believes the future is going their way, unlike the Germans who basically believe they would lose out in the long term. They don't have a insane collapsing Austro-Hungary as an ally constantly dragging them into conflict. And nuclear weapons exist.
Note that Russia's oil does not help China here: the infrastructure in Russia only allows export via European ports, not Asian ports and not over the Russia-China border, though with the construction of a few 5,000-km-long pipelines, I guess that could change in a few years if Russia cares enough.
I believe Russia and China signed agreements for building gas and oil pipes.
https://en.wikipedia.org/wiki/Altai_gas_pipeline
https://www.reuters.com/business/energy/putin-says-oil-pipel...