Because it's an easy source of glucose. You have to process something into glucose, it isn't a naturally occurring sugar in large quantities in plants, so since sucrose is literally just glucose and fructose stuck together, why wouldn't you? It's one of the many ways it's made at industrial scale efficiently.

This short article describes the process and the various feedstocks commonly used, if you're curious.

Edit: While the most dominant feedstock is starch, processed sugarcane (i.e. once the sugar has been largely removed) is used as a feedstock for glucose since it would otherwise just go to waste. Interesting research article here.