China is in the “crawl, walk, run” process when it comes to aircraft carriers and has not yet reached its desired endpoint where it would buy several of the same design. In other words, they aren’t yet at the level of a U.S. Super Carrier and won’t be for another iteration or two of designs. Their first carrier, the Liaoning, is an ex-Soviet Kuznetsov-class that they acquired from Ukraine and completed. Their second carrier, the Shandong, is a modified Kuznetsov. So the first thing about your assertion is that it is wrong, China’s first two carriers are the same base design. Since China wants to move on from the Kuznetsov’s STOBAR design to CATOBAR their third carrier (not yet in service), the Fujian, is a new design.

We don’t yet know what the fourth Chinese carrier will look like. We know China aspires to nuclear propulsion and the Fujian is not nuclear powered. Will the fourth carrier be a new nuclear propulsion design? Will it be a Fujian-class modified for nuclear propulsion? Will they build a second conventionally powered Fujian-class ship before moving on to a nuclear powered one? We don’t know for sure at this point.

As for the U.S. you are comparing a navy that has been building carriers since 1920 and has over 103 years of different carrier designs behind it with a Navy that just commissioned its first aircraft carrier a decade ago. So you would have seen this rapid evolution earlier in the U.S. development history. As recently as the 1950s the U.S. would build 3 or 4 of one carrier type then move on to another design. Even that is misleading as there are sub-classes. The 4 carrier Kitty Hawk Class is actually 3 carriers of the Kitty Hawk design plus a single carrier, the John F. Kennedy, that is considered a distinct subclass. This is similar to the Shandong being an improved version of the Liaoning/Kuznetsolv-class. The U.S built the one in class USS Enterprise, the USN’s first nuclear powered carrier to prove out nuclear propulsion. With the Kitty Hawk and Forrestal classes having proven the design for Super Carriers, and the Enterprise having proved nuclear propulsion on Super Carriers, the USN moved on to the Nimitz class.

The first 3 Nimitz class are really a distinct subclass with the remaining 7 having an updated design. The USN considers them a single class, but really there is the Nimitz-class and the Improved Nimitz-class. The Improved Nimitz were all delivered after the end of the Cold War, and so the U.S. lacked a threat that might have quickly forced another new design. That would wait for the Ford-class that has only recently entered service.

Gerald R Ford (Ford Class) and Harry S Truman (Improved Nimitz Class)

Even within a ship class or subclass in the USN there are improvements incorporated in newer ships so that each ship is somewhat unique. Sometimes the changes are retrofitted to the earlier ships during a major maintenance period, sometimes they are structural changes that can not be backfitted. In any case, the ships are not just cookie cutters of the same design but incorporate improvements from ship to ship. The USN just chooses not to highlight that via changes in class name.

Update (11/16/2023): Fixing a couple of details, but it doesn’t change the overall answer