Because ship-based SAMs like the HHQ-9 have a maximum range of 300km.
Opposing aircraft can detect ships at 400km+ and have antiship missiles launched at even greater distances.
If Chinese fighters stay within ship-based SAM coverage, sure, the fighters can be relatively safe. But the ships below will face multiple waves of incoming antiship missiles, and eventually there won't be any ships left. Then those fighters fall into in the sea because there isn't anywhere to land.
Chinese fighters have no choice but to leave the envelope of ship-based SAMs.
First of all, ships in any large scale fleet are well dispersed in a defensive layered formation. Thus the distance between the carrier at the center and destroyer on the perimeter could be more than 100 KM.
Which means for enemy planes to attack the carrier, they have to come much closer than 300 KM from the nearest destroyer.
Moreover, HQ-9 is not the be all end all of ship based air defense. China recently unveiled hypersonic DF-1000 which can target planes from 3000 KM away. China is also developing 1000+ KM SAMs. These will likely be navalized and put into destroyer once they are mature.
Ships can always bring bigger missiles and take out targets from a longer range. The limitation is never the size or range of the missile but the ability find and track targets with accuracy.
This is where China's advanced AWACS radars come in with their ability to track targets all the way to 600+ KM.
Moreover your 6 squadron of around 120 planes can only carry maybe 250 anti-ship missiles, This is easy task for 20+20 destroyers and frigates to defend with their SAMs.
Once these planes go back to carriers to rearm and refuel, Chinese fleet can track them and launch their hypersonic missile attacks on those carriers. So, its not true that Chinese fleets will keep defending wave after wave of enemy attacks without firing back.
In the simplest possible terms: ships are slow, planes are fast, and the ocean is big
Missiles are even faster. Planes can only fly for couple of hours and then must return back to their carriers, which is slow, easy to track and easily sunk with hypersonic missiles.