IMO, scientists conveniently preclude one possibility.
Maybe Earth is the only planet in the Milky Way to support sapient life. Somebody will respond that this is improbable, but there is no reason to categorically dismiss it, is there?
Also, a scientist should be open to all eventualities, and not rule things out based on his or her own subjective bias.
As an example, if a geologist devises a theory to say that giant burrowing snakes in the mantle causes earthquakes, then of course the geological community will literally laugh out loud at that suggestion. However, this is only based on the evidence at hand and inductive logic, not personal feeling.