If a central bank carried out monetary policy that consisted of multiplying each person's wealth by a common factor, I would find it plausible that all prices would get multiplied by the same factor and there would be approximately no real effects.
However, this is not how central banks carry out monetary policy. They print money and engage in market transactions that target certain prices. To me it does not seem plausible that this has no real effects, in the long run or the short run.
My question is, why do many economists believe that money is neutral in the long run? Do they take the quantity theory of money as proof that money is neutral in the long run? Is there particular empirical evidence, or influential papers that have convinced many economists that money is neutral in the long run?