Why do the outer parts of wings generate less lift?
The outer wing section generates less lift because flow around the wingtips reduces the pressure difference between both sides. This effect dies down quickly when moving inboard, but will still reduce lift somewhat at the wing root.


Screenshot of the Stanford Aero wingcalc page. This is a Java page for which you need to add http://aero.stanford.edu/wingcalc.html to the exceptions list in Java. Then you can change aspect ratio, wing sweep, taper ratio and twist. Here the lift distribution for an unswept, untwisted rectangular wing is shown which has a local lift coefficient of almost 1 at the center. Due to the low aspect ratio, wing efficiency is still at 99%

A rectangular wing can be optimal if stall characteristics and ease of construction are included:


The lower lift coefficient at the tip ensures that the stall starts at the wing root, resulting in a docile stall behavior without a sudden rolling motion.
The constant chord allows to use one single jig for building all ribs of a wooden wing.
The shorter root chord of a rectangular wing requires a heavier spar, but when wings are braced and the root bending moment is low, the weight impact is negligible.


If the goal is a simple to build aircraft with forgiving stall characteristics, the rectangular wing will be an efficient choice, especially when bracing is used. Only when performance is most important will a rectangular wing be less efficient than a trapezoidal wing.