Two out of four Supply class vessels are in reserve, they are not retired. USN can bring them back into service if they are needed. In any case the USN of course operates an all nuclear powered carrier force, making the need for large combat supply ships less pressing. The mothballing of the two Supply class vessels followed about five years after the last conventionally powered US carrier, Kitty Hawk, was retired.The US Navy equivalent of the 901, Supply class also only has two ships commissioned. Four were built but high costs to operate have forced the US Navy to retire them early. The PLAN also has 8 active Type 903/903A's.
More significantly (as mentioned above), the USN also has 14 very large dry stores and ammunition ships (T-AKE) and 16 fleet replenishment oilers of similar size to Type 903. Of course their operational commitments are global, while PLAN has arguably regional requirements. But either way PLAN does need more large auxiliaries if they wish to support a fleet of conventionally powered carriers, esp including ships such as CV-18.