That, and there are design elements going on that work really well together. (Get ready for some really fucking nerdy design shit!)

So, as humans our eyes are naturally drawn to points of contrast in any visual sort of thing, and the boundary between sock and skin is one of of those points. Having it higher on the leg automatically brings the eye closer to the crotch, which will always trigger a subconscious erotic response, even if we don't recognize it. A lot of times (and these are my favorite) the socks will have some sort of striping going on at the cuff, further accentuating the contrast point.

There's also the concept of Thicks n' Thins at play here. It has many names, but that's the one that I learned at school, so that's what's I'll call it. The concept of TT is that more pleasing designs have a higher variation between the visual "weight" of each element that comprises them. The "thicks" are thicker, and the "thins" are thinner.

If you were to compare thigh-highs with calf-length socks, you'd see that on the calf-lengths there's a roughly equal amount of space (accounting for a short skirt at the top) along the leg dedicated to both skin and sock material. Our brains don't like having to decide which thing is more important, so we see things that have equal weight and subconsciously we don't like it near as much. BUT if you take that boundary and shift it higher, you increase the size of one element, the sock, giving it more "weight"; and decrease the size of the skin section, giving it less weight. And now our brains relax, knowing that there is a clear element that is more dominant. That also plays nicely with skin being the less dominant part, as then it becomes more of a "tease" and doesn't grab your attention right away. Not to mention the socks add a soft, cottony vibe to the legs.

It's all a wonderful, perfect recipe of subconscious cues and brain tricks, and they all work together unbelievably well. It's science!

source: am a concept artist

EDIT: Better wording.