Because the finished features are only part of what you're paying for.
Site preparation, excavation, grading, drainage, base materials, foundations, utilities, electrical, irrigation, structural requirements, concrete specifications, material selections, access, demolition, and construction coordination can all affect the scope.
Two proposals may show similar-looking finished spaces while representing very different levels of construction underneath them.
Ask what is included, what isn't, what materials and specifications are being proposed, and how the contractor evaluated the property before arriving at the price.
Compare the scope and construction behind the finished design—not simply the bottom-line number.