That’s because valid succession is not enough to guarantee valid Ordination. Every Sacrament has five required elements: minister (which is where succession comes in), subject, matter, form, and intent. If any ONE of these is lacking, the sacrament is not valid. This is also the teaching of Anglican theologians (such as Francis Hall). In the case of Orders, only one of the five requirements has anything to do with valid succession.
The Anglicans (after being heavily influenced by Calvinism) lost their Orders (which had been recognized as valid for decades) through theological changes resulting in a lack of valid intent (and probably of valid form). A “valid” Bishop (which the Anglicans had in their early days) does not ordain someone without ALSO having valid intent and form, and these were determined to be insufficient. (In the case of Barbara Harris, the ordination would further be invalidated by lack of proper subject.)
Through the Oxford movement, the Anglican Church has regained much of what it had lost, but not in time to avert disaster. By the time they recovered, there were no validly ordained Bishops (ordained with valid succession AND valid form and intent) remaining. All of their validly ordained Bishops had died out, and the Anglican Church was left with a group of laymen in Bishop’s cassocks with valid succession but invalid ordination.