Here's why:
* Mew is a Mythical Pokémon: It's not found in the wild like other Pokémon. You typically get it through special events or specific game mechanics.
* Ultra Balls are for regular Pokémon: They are effective for catching wild Pokémon, but not for acquiring Mythical or Legendary Pokémon.
How to get Mew:
* Special Events: Nintendo sometimes offers events where players can obtain Mew through special distributions. These events usually require connecting your game to the internet or using a specific code.
* Trading: You can trade for Mew with other players. This is a common way to acquire it if you missed out on a distribution event.
* Specific Game Mechanics: Some games have unique ways to obtain Mew. For example, in Pokemon Red and Blue, there's a glitch that allows players to duplicate the "MissingNo." Pokémon, which can be evolved into Mew.
In short, you can't catch Mew with an Ultra Ball in the traditional sense of a Pokémon battle. You'll need to rely on special events, trading, or unique game mechanics to acquire this elusive Pokémon.