Because they r not native to that area. For example if I had a pond on my property .5- 2.5 acres obviously man made and added wild caught fish and the fed themselves they would not be wc parents are offspring not. Look at the south Florida waterway as a big ass pond with introduced species all offspring are f1+ they r not wild caught even 50 yrs ago of all the introduced jags wc or captive breed that we're introduced once they produce offspring ( WC x CB) the offspring are not wild caught any more.