I had the same question earlier, but thinking about it more, bringing up the vote may force some politically awkward position. It may be that Gaetz and one or two others are the only one who actually wants to Oust McCarthy, so if it takes five to initiate a coup nothing happens. But if a single Republican files the motion then every Republican is forced to choose a side, and those who would otherwise not mind the status quo may feel the need to vote against him to prove their Conservative bonifides.