Why don't Chinese or Russians copy YF-23?

We all know that China and Russia jumped right into development of 5th Gen aircraft, be is J-XX (J-14) or PAK-FA. The first reaction is that those aircraft won’t be as stealthy as F-22 is today..

My question is – why don’t they simply externally copy the shape of YF-23? The thing was said to outperform the YF-22 in terms of stealthiness, that would mean it must be at least +/- comparable to F-22A. Would not it be a logical thing to do rather than spend thousands of days developing something that you can copy in three months?