I think the idiom by itself is good, and that could be the reason why LLMs prefer it: a hypothetical test groups liked it.

The problem comes from that LLM prefers it way too often.

And I suppose it would be a bit difficult to change it to actually be good: even if it just used it once per session, it might still choose to use it on every session, as it cannot see how much an idiom has been used in other ones.