Why do I have to clone a disk from live USB?

Your question is unclear, and there are times when you don't need to do this. Assuming you are talking about the common case of cloning a disk which is operational (eg cloning your current windows setup), the answer is this -

Cloning a disk takes a while. During this time the contents of the disk will very likely change, even if only due to background processes. This means that the file index (typically stored at the start if the disk) will be out of sync with the actual filesystem layout - which can cause corruption, instability and data loss.

I comment that cloning a disk is a block operation - ie the whole hard drive is treated like a single file - this is different to copying files within the OS because a block copy has no knowledge of the filesystem layout stored inside the block.

Please read the following thread as well: How does modern disk image software make accurate images of drives in use?

How does modern disk image software make accurate images of drives in use?

Cloning offline media (using "live operating systems" on a USB stick p.e.) is more reliable than cloning. Furthermore imagine cloning to a tape drive where you would need to add all intermediate changes of your operating system as well. Each write operation of the running operating sytem would extend the length of the clone on tape. Using a "live operating system" would prevent the clone on tape from being extended by write operations on the media in question.