You are both right and wrong in the same sentence. Arbitrage is absolutely not risk free for all the reasons you then point out. In the time it takes you to transfer your coins, the gap can close or the price fluctuate wildly. Exchange, deposit and withdrawal fees can easily eat all your potential profits and more leaving you at a net loss, even if the price gap remains stable. Low liquidity can stop you from selling altogether.

Yes, there are profits to be made with arbitrage, but it's usually by bots with fiat and BTC already sitting on a couple of exchanges waiting for opportunities to arise. By the time the average trader gets involved, the gap is usually closed.