Why do insects (and spiders and centipedes) have legs that are angled acutely upward then downward to meet the ground whereas mammals large and small all have straight legs that curve only slightly when walking or running? Why are they so different in morphology when they serve exactly the same function, i.e. load bearing and level attitude keeping of the central torso when in rest and motion? If the larger mammal's leg is evolved to favor large ground clearance for eyes to better spot threat/prey, then how come very small rodents also have straight legs when they should evolve to favor rapid gait because they are so small that they can not stand above grass or most obstacles in nature?
Evolution is based on chance. AFAIK, the ancestors of arthropods and vertebrates diverged before legs really evolved. From there, whatever bunch of random changes that improve fitness get adopted. (And not all arthropod legs fit that insect architecture: for instance millipedes.)
You're up against the square/cube law. That is, body mass increases as the cube of size. That means that the larger an animal is, the stronger splayed-out insect-style legs would have to be, and the more muscle power is needed to support & move them. For example, try doing a pushup with your arms extended out to the sides: it's a LOT harder than the normal arms underneath you one, isn't it?