Speaking from the view of the adoptee the clumsy language used by Social Services when I started in 2005 was enough to put many pre 1975 adoptees off and arguably more so the male adoptees. The initial meeting was often described as a 'counselling session' with a social worker, it's understandable that many questioned why they needed counselling and didn't proceed.
Some adoptees don't even know they are adopted, some only find out by accident and even those who do know may delay looking whilst their adoptive parents are still alive or have no desire to look.
My bio parents died many years before I started searching, I found maternal half siblings literally hours after the first meeting with social services though they made it clear they did not want contact. My paternal half siblings were more difficult to find, in fact they found me really through a post here on Rootschat.
They are the complete opposite to my maternal siblings and we keep in touch (six are full sibs to each other), two committed their memories & photos to paper to publish a book 'Our Lee',175 pages about their lives. Lee passed away in 2019 and along with Our Paul who died in 2017 are the brothers I never got to meet but through the book are no longer strangers. We did manage to get a copy to another half sibling who I found just a couple of years ago. Twenty five years and 12 half siblings later maybe I should think about a book of my own.