Why are PTAS-Reductions used to establish APX-Hardness?
Let me correct two misconceptions in your question:


$PTAS$-reductions are approximation-preserving
$APX$ is the class of problems that have constant-factor approximations


The big open question isn't whether $APX$ problems have constant-factor approximations (by definition, they do), but whether they have polynomial time approximation schemes. The meaning of $APX$-complete is "As hard as any problem in $APX$ to find a polynomial time approximation scheme for". A $PTAS$ from $A$ to $B$ reduction allows any polynomial time approximation scheme for $B$ to be used for $A$.