Why are early wing planforms mostly rectangular?
The benefit of tapered wing lies in its proximity to the elliptical lift distribution while retaining much of the structural benefit of a rectangular wing. But we owe this knowledge to a few things:


The Kutta-Joukowski Theorem: published in 1906 by Nikolai Y. Joukowski and influenced a great deal by Martin W. Kutta.
The Lifting Line Theory: published in 1919 by Ludwig Prandtl and inspired by the work of Frederick Lanchester. The term induced drag was coined by Max Munk, a colleague of Prandtl in 1918. Although Lanchester had published some results on aspect ratio and finite-wing aerodynamics as early as 1907, they weren't taken seriously by his compatriots and had very little impact as a result.


The Wright Flyer flew in 1903. By all accounts, the Wright brothers designed it through trial and error. As a result, the insight of tapered wing couldn't have been known to them during the Flyer's design and through much of the First World War.

In any case, most of the early airplanes were bi/triplanes with external struts being their structural members. The drag from the struts would've overwhelmed any benefit gained from tapered wings.