Question

Why does everyone choose frisk over compoundeyes to find items when clearly compoundeyes is much more effective?

Comments

On which Pokémon?

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Answer 1

Because they serve different purposes?

Assuming you're talking about wild encounters and not competitive battling, Compoundeyes increases the chances of finding an item on a wild Pokemon from 50%/5%/1% to 60%/20%/5%, but you still need to use something like Thief or Covet to check if the Pokemon is holding an item.

Frisk will tell if you a Pokemon is holding something, so if it isn't, no message will appear at the beginning of the battle & you can just run away.

Answer 2

If you have a Pokemon with frisk and the move(s) trick/switcheroo, you could take some rare items that could be useful. Or in wifi battles if you have frisk and trick/switcheroo, you could destroy your opponents strategy.
Hope this helps!