Why people ask it
Because a miracle sounds like a claim that physics is wrong, and physics is very obviously not wrong.
Short answer
Natural laws describe what happens when nothing intervenes. A miracle claims an intervention, which changes the conditions rather than falsifying the description.
Deeper answer
If I drop a ball, gravity describes what follows. If I reach out and catch it, I have not violated gravity — I introduced a factor the description did not include. Miracle claims have the same logical shape with a different actor.
There is a further point people often miss: recognizing a miracle requires that nature be regular. If anything could happen at any moment, nothing would stand out as exceptional. Miracles depend on consistency rather than denying it.
The deeper issue is usually not evidence but starting assumptions. If someone holds that no possible evidence could count for a miracle, then the position is being held prior to the evidence — and it is worth both parties noticing that.
Common mistake
Claiming miracles are common. Rarity is part of the concept. If they happened routinely they would just be physics.
Conversation-ready response
""What evidence would you accept for a miracle? If the honest answer is none, then we are discussing an assumption rather than the evidence — and that is fine, I just want to know which conversation we are having.""
Source notes coming soon.