Because most stories about "princesses" attempt to date themselves back to a time in the world where clothes were a kind of architecture. Nobility wore rich fabrics, carefully cut and finely worked. The resulting attire obscured virtually every portion of their bodies except hands and face, creating an image that only a select few could aspire to.
Of course that no longer applies to current fashions. In this day and age, the body itself is the major architectural structure. Shaped and molded by pumping iron, rigorous dance, endless hours on treadmills, starvation, distance running. Smooth, tanned skin over well-muscled flesh is the most appealing garment now.
But we don't have modern-day princess stories. We don't have modern princesses. The concept just feels too strange, it wouldn't work.
History. In the past, that was the fashion - and the requirement - for girls. Girls couldn't wear pants until pretty recently in the grand scheme of things, and most of the princess stories we tell take place in the past.
More generally, I, as a girl, wear dresses pretty much every day, because I think they're more comfortable. I'm a dancer. I can move in dresses more than I can in jeans or shorts. Suits make me feel like a business person and that makes me scared I'll have to work in a cubicle, and I don't wear bikinis because I believe modesty is a lost virtue.