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I think it depends on the name. The popular names you mentioned are likely to get nicknames if they are constantly around other individuals with the same name. Long names are given nn's because it is cumbersome to say Theodore or Genevieve all day long. I think other names don't need nn's but likely get them anyway due to the laziness of individuals in saying the full name.
I don't really know why I like some names to stay full and other to get nn's, but it probably has something to do with the nn they are given. I really dislike the nn's Jim and Joe, but I don't mind Liam and Will.