Thighs are much trickier to work with as they’re rotating whereas the upper arm is static.
Through pedalling the angle of the thigh rotates a lot. This means texture in this area usually ends up creating more drag. There are some instances that a small amount of texture could be beneficial, but it wouldn’t look anything like arm ribbing.
If you see anyone doing ribs on the leg run away - it doesn’t work and it’s a pretty good indicator they don’t know what they’re doing when it comes to aero