Why can't we have 4 letter usernames? 😭

I know the line has to be drawn somewhere, but I feel like that somewhere should be 3, not 4. I think there are so many obvious 4 letter usernames, like Sage, Sela, Luna, Zero, Dawn, Nova, Meli, Lily, Faye, Nyxx, Neko, Lina, Loki, Cleo, Kora, Vega etc.