Because the Earth is round. Well, curved, at any rate. You think you're walking in a straight line, but you're actually walking slightly down with every step. That's because a straight line forward would be tangent to the Earth's surface (or secant to the atmosphere. What you're trying to walk actually approximates an arc of a circle.

However, if you're talking about EVA -- a spacewalk -- you can generally move in something like a straight line, as long as you're not too close to a massive object. Otherwise, spacetime itself will be curved, and all your hopes of walking a straight line will go bye-bye.

And that, Your Honor, is why I couldn't walk a straight line when the state trooper pulled me over. Honest.

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