Many airliners have a distinct unswept part at the wing root at the trailing edge. See for example this image from Wikimedia of a B737-400.
Other examples include the B777, A320, Embrear 145, and many more. Virtually all airliners have at least some degree of less sweep at the wing root.
Why is there a part of less sweep at the wing root (to facilitate engine mount; Aerodynamic reasons)? Why does a completely unswept part seem to be a preferred choice for this part of less sweep, as opposed to any other sweep angle than zero?