Here's why:
* Sieves are designed to let water pass through. They are porous, so water cannot be held within them.
* Trying to carry water in a sieve is a pointless endeavor. The water will simply drain out.
This idiom is used to describe situations where someone is trying to do something that is inherently impossible or very difficult to achieve, and their efforts are likely to be unsuccessful.
Examples:
* "Trying to get everyone to agree on a single solution is like carrying water in a sieve. There are too many different opinions."
* "She's trying to save money by cutting back on her expenses, but it's like carrying water in a sieve. She's just not good at managing her finances."
The idiom emphasizes the futility and pointlessness of the effort, implying that it's better to focus on more realistic goals.