In Gen I, the battle mechanics were different. Psychic Pokemon were immune to the Ghost type, and that's why Cherry's village raised Psychic Pokemon to counter Ghost types. Psychic does x1 damage to Ghost in all generations, but in Gen I they had the benefit of immunity, and also the Gastly evolution line was the only Ghost type in Gen I, to which Psychic was super effective against due to its part Poison typing. That's why Psychic Pokemon were used.
You also might be mixing up Ghost & Dark types, the latter of which is immune to Psychic; Dark didn't come into Pokemon as a type until Gen II.
Ghost type battle properties - Bulbapedia