that's an interesting question europ2,
I think there are several reasons for so many panels:
1. Design: camber/foil varies across the span, canopy does not have the same depth profile or shape at the wingtips, shoulders, and center. Need a bunch of panels to create the shape.
2. Durability: seams pick up and redistribute loads so the light cloth doesn't bag out as quickly, letting kites last longer. Seams can also build in some give to the shape in a controlled way, and even stop or slow the spread of rips across the canopy -sometimes, at least.
3. Quality control: imagine if you depend on two chord-wise seams each side of the kite to create the profile. If both seams were out a millimeter or so in the same direction (assume 50% probability), you get a 75% probability of a kite that would not fly straight (because you would only have a 25% chance that the seams on the other side were both out in the opposite direction to compensate for the error). If you have numerous seams each side, regression to mean gives you a much greater chance of achieving your designed shape with a given level of accuracy in sewing (unless you let the same incompetent sew all the seams in the same direction
Now consider again how the profile varies over the span: you will see a benefit from multiple seams in each "zone" of the profile. Therefore the high number of panels.
I don't know for sure if this is how they do it in the factories, but it makes sense...
But your question is still a good one!
There are other solutions to flight than the known, obvious ones, look into the facetmobile for example, an experimental plane that looked a little like a stealth fighter, and flew very well, with low horsepower and no electronic controls, despite (or because of!) a total of only about a dozen major planar panels (plus 2 vertical rudders) making up the entire surface.
Now I don't suggest a homemade UFO-stealth kite is a panacea or even feasible, putting loads onto strings and cloth will give you curves, not facets!
That unusual plane is just an example to point out that there are a lot of possibilities besides the conventions of sailmaking and "normal" airfoils that may come to light with more research and experiment.
LEIs without struts, aren't really pushing the design envelope imho, just moving around inside its less known corners.
Many more possibilities are there, in both shape and construction, I think.
Here's to discovering more fun, economical and durable ones!