Only because the trees are above the centre of your camera's field of view.
Point your camera higher and they will curve outwards.
You can use a fisheye lens to make the earth look spherical (point the camera below the horizon) or as if we're on the inside of a cylinder (point the camera above the horizon). Incidentally in all the pictures of mountaineers summiting Everest with the curvature of the Earth visible the apparent curvature is entirely due to lens distortion.
(not a flat earther but you need to go higher than Everest to be able to see the curvature of the Earth. Apparently it was visible from Concorde which flew higher than other commercial airliners with a flat horizon visible).