Mining is not marketed as risk free though. I have personally made some money through automated inter-exchange arbitrage but I ended up concluding that the risks outweighed the rewards at current time (maybe intra exchange is less risky).
most of the inter exchange arbitrage opportunities are eaten up by the fees and because there can be a time delay across exchanges it also adds risk. The intra trading (especially in my case ) has those risks minimized. there are no fees on cex and the trades happen very fast.