The world is round but the atmosphere and geological phenomenon are much different. 'Straight line navigation' is impossible after you cross the Grand Line, so you have to use the log poses to navigate.

The islands arent in a straight line, but spread apart. No matter which direction you start from, you'll be directed to a series of islands which you must cross to get across.

Think that the Grand Line runs all the way around the globe, and now imagine that you cross the Grand Line and enter the hemisphere which isn't reachable unless you cross the Grand Line. (Keep in mind that the Grand Line stretches all the way around the globe. You can think of this area as the 2nd half of earth)

The 'red line' envelopes a region at the center of this hemisphere. So no matter where you enter the Grand Line from, the log poses will direct you to the central region i.e the Red Line. Hence you cannot reach the Red line without getting past the Grand Line and several islands first

Hope this helped.