Heres my theory:
I think Hank Pym was probably the 2nd official super hero in the MCU after Captain America. He started out as an upbeat, idealistic scientist who became a secret SHIELD agent using his tech. He and Jan then went on missions as Ant Man and Wasp. It could possibly be that they along with Howard Stark and Peggy Carter created the original Avengers Initiative. At some point, Jan was killed maybe accidentally by Hank during a big mission. Feeling responsible, he gave up his Ant Man alter ego and quit SHIELD. This is why his history isnt widely known by Tony etc. It was probably only a brief time he acted as Ant Man. Decades later we'll see him as the older, more pessimistic scientist who despises super heroes like Iron Man & Co who have been more successful at what he had tried to do.