Some do. Despite my attempts, I've never successfully dared a cis male FA to wear a skirt...
As your picture very well illustrates, FA uniforms are a marketing billboard for the airline. Their employer wants them to look a certain way. And most of these employers want skirts and heels.
There is no practical reason for wearing skirts, heels, and nylons aboard an aircraft. The most practical option, both for normal flight and for emergency situations, would be pants and a hoodie. Fabrics are probably a bigger problem than skirts: practical materials would be a merino wool underlayer and a loose rougher wool outer layer with pants, with mid-calf or taller leather footwear. Note that Nomex isn't necessary - it's comparable to wool in fire resistance, but loses on heat regulation and odor resistance.
Early FA attire was almost practical, with flat shoes and natural fabrics, but still with skirts, since it was unusual for high-class women to wear pants in the early 20th century. Stockings these days were also made out of natural fabrics and didn't represent a danger.
Since then, high heels became more popular, and fabrics got worse. Many airlines want bright colors in their branding, which are much easier to achieve and maintain with synthetics than with wool or silk. Synthetics are easy to clean and hard to damage through abuse or improper washing. No shrinkage, no bleeding, no stains, and cheap to replace when hiring/firing.
Most current FA attire is impractical and impairs performance both in normal work, if heels or tight skirts are worn, and in case of emergency. That said, lungs remain the most vulnerable part of the human body. Synthetic fabrics will destroy the skin underneath if exposed to fire, but lung failure prevents the grimy consequences of plastic burns by killing one first. So there isn't a lot of fatalities due to burns, compared to smoke inhalation.
Are more practical or protective uniforms necessary? Depends. While air is the safest form of travel per mile, for professionals averaging thousands of miles per day, accidents do become a non-negligible factor in mortality. With an accident rate of ~1 per million departures, it takes 74 flights per year for an air accident to be as likely as a major driving accident in Europe. That's a lot for a passenger, but on the low end for cabin crew. Flight attendants aren't entertainment; it's a physical job, with a key role in the safety of the passengers aboard.
So it would probably be worthwhile to modernize FA uniforms. The heels, for one, impair performance to the point that they must be removed in an emergency, not to mention long-term health concerns. Gender norms have long since changed towards more unisex clothing. But there isn't any political or regulatory push for such a change at the moment.