It’s all about marketability. A thin slab phone is more marketable than a thick slab phone. A (thick) tri fold is more marketable than a thin slab phone. I’m sure if they find a way to make the trifold even thinner they would though at that point idk where they would put the USB port.
The tradeoff for a really big screen is a thicker device. That’s more valuable to more people than the longer lasting battery. At least in the eyes of the manufacturers. I’m sure if bigger batteries sold better there would be more (any) options. The issue is not enough people care.