It takes a long time to design and build modern aircraft carriers, then you've got to develop the doctrine and good habits to put all of it to use.
The modern world isn't WWII, when you could just throw some aircraft up there and let them shoot it out. Modern aircraft combat amplifies even small differences in ability; a squadron of F-22's might shoot down a squadron of F-15's before the F-15's diagnostics and support could even spot them. I've read anonymous anecdotes of US vs UK wargames in which the last gen US fighters (pre-F22) would wipe out all of the UK aircraft before being detected. Anonymous anecdote, but still.
The US also has decades of anti-guided and anti-ballistic missile research and experience. Sure the pure technology can be copied quick, but deploying it to your sea fleet and again developing the proper doctrine and support systems for actually using the stuff takes time.
I'm not familiar with the Chinese submarine situation, but the US takes that area very seriously as well.
And the problem is that you've got to be strong on all three fronts. Losing largely on any one - surface ship, air combat, submarine - means that your entire fleet may be sent to the bottom of the sea in mere hours... or minutes.
Note that the Chinese military leaders understand this, and are focusing their efforts on asymmetric defensive warfare - at least as pertains the US. 'Carrier killer' missiles, sufficient numbers of cruise missiles to overwhelm US naval defenses, submarines. These are relatively cheap and enough to keep the US off China's back (indeed the US is rumored to be keeping all carriers far far away from Chinese coastlines due to the carrier killer) - but it's not enough to achieve anything like parity.
Of course I'm just an armchair guy on the internet. I don't have any inside knowledge, just what I've gathered in my reading.
"In January 2007, it was reported that the F-22... [amassed] a 144-to-zero kill ratio during "Northern Edge" air-to-air exercises held in Alaska, the first large-scale exercise in which the Raptor participated."
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"I can't see the [expletive deleted] thing," said RAAF Squadron Leader Stephen Chappell, exchange F-15 pilot in the 65th Aggressor Squadron. "It won't let me put a weapons system on it, even when I can see it visually through the canopy. [Flying against the F-22] annoys the hell out of me."
Lt. Col. Larry Bruce, 65th AS commander, admits flying against the Raptor is a very frustrating experience. Reluctantly, he admitted "it's humbling to fly against the F-22," - humbling, not only because of its stealth, but also its unmatched maneuverability and power.
Edit: And found one of F22 v Eurofighters; both are GenV fighters:
Two other German officers, Col. Andreas Pfeiffer and Maj. Marco Gumbrecht, noted in the same report that the F-22′s capabilities are “overwhelming” when it comes to modern, long-range combat as the stealth fighter is designed to engage multiple enemies well-beyond the pilot’s natural field of vision – mostly while the F-22 is still out of the other plane’s range. Grumbrecht said that even if his planes did everything right, they weren’t able to get within 20 miles of the next-generation jets before being targeted.
“But as soon as you get to the merge…” Pfeiffer said, referring to the point at which fighters engage in close-up dog fighting, “in that area, at least, the Typhoon doesn’t necessarily have to fear the F-22 in all aspects… In the dogfight the Eurofighter is at least as capable as the F-22, with advantages in some aspects.”
>I've read anonymous anecdotes of US vs UK wargames in which the last gen US fighters (pre-F22) would wipe out all of the UK aircraft before being detected.
Now the UK's bought into the F-35 and what a mess that's turning into.
That is seems like very long. Why would it take three decades?