I could be mistaken on the form of sea transit. I just know it gets refined in Canada (which also seems like a waste: why not refine at source?).

Sea transit is cheap, sugarcane grows better in the tropics than beets in cooler places. Harder to harvest below-ground stuff.

Canada used to subsidize/protect sugar beet producers (to keep them competitive), but largely got out of it.

Maybe that’s why we still refine it: because the plants still exist.