Because SQ and CX have no narrowbodies and therefore it would be unfair to compare a HU 737 against a SQ 777. Even comparing widebody hard product the HU A333 are as good as Singapore's A333 in economy, and probably better in business (staggered 1-2-1 vs 2-2-2).
Although short haul narrowbody operations are the backbone of an airline, they do not represent the airline's best product and therefore should not be judged accordingly. This is not only true for HU, but also other airline I mentioned like OZ, MH and TG. When comparing long-haul products of HU and other competitors, HU's is definitely on par with 5-star standards.
Given that they have, by my quick research/math, more 332/3s than any other wide body model, I question your "outlier" qualification.
They have 16 A332's with "enhanced product" (lieflats and PTVs), and 4 A332s, 5 767s, 6 A340s, 10 747s and 10 777s (total 35) with dated product. That's around 32% compared to Hainan's 100%.