Because the USSR in the 1980s was probably the last time America was seriously threatened at sea in any real terms.
Relative to the U.S., the Soviets had overmatch in anti-ship air and undersea platforms, and an extensive satellite surveillance network, which meant carriers had to operate in FCTFs to survive. The Chinese only have one of these, and it's rather unlikely they will achieve the former two before the 2040s when the USN's submarine cliff arrives, so call it 10-20 years of non-investment before the PLA becomes a Soviet level threat to the USN i.e. regional sea control with advanced aircraft, equivalence or superiority in submarine platforms and a real-time satellite observation/targeting system over the Pacific.
That assumes no real changes in production or delivery of ships to DOD beyond the current schedules and maybe a few more delays for USN.
The U.S. as of now is able to keep China as a modest threat to Japan and Taiwan by spending less than 4% of its GDP on DOD. In the 1980s it was closer to 6-7% of its GDP, so about double what it would be now. China can probably still keep up with a $1.5-1.6 trillion defense budget from DOD, but it would be much harder to pace that budget, at least without cutting into their civilian economic investments.
If the U.S. gets closer to the hull cliffs without meaningful change in ship schedules, it will simply start ratcheting up its defense spending for a presidential term or two, and the Chinese will find it harder to pace (they have a weirdly sluggish MIC in their own right), and this would probably translate to getting the LHAs, carriers and SSNs sorted out within a couple years, and back on track for long leads within 5 years, in real terms. I'm not sure there's much that can be done in a decade of high investment quickly in any other manner for shipbuilders since U.S. yard expansions are somewhat glacial. They can definitely send out ships as fast as the PRC though.