I mean the Sun melts the Ice, so why can't it hurt the Ice Types little by little? Especially on Pokemon like Baniricchi and Furiijio who are Ice cream and a snowflake(B/W Pokemon)?
Sun and rain don't harm Pokemon by default, but sandstorms and hail do.
It's just a decision the game developers made. Not everything is 100% logical, otherwise the games could get even more complex.
Because a One-hit KO from a weather effect would be a terrible idea...?
(See "Shedinja" for more details)
I know what your thinking, sun should melt ice but
sunny day was mainly maid to make fire moves stronger. it also changes the move solar-beam so it doesn't have to drain energy from the sun.