The reason is that they are made with the same steel used by doctors during surgeries . That is why it is called surgical steel, which is also the most in demand for manufacturing body implants and piercings.
Surgical steel is a variation of steel (iron + carbon) also composed of an alloy of chromium (which prevents wear and corrosion) and molybdenum (which gives it hardness). In modern jewelry, nickel has almost been eliminated from the alloy, which is the element that can cause problems.
That is why it is recommended for summer, since in addition to not causing any type of allergy problems, it is a material that is not damaged by sea or pool water. The list of its benefits is long: it holds up much better over time than other materials such as silver and gold, and with the enormous advantage of requiring much less maintenance and being more economical.